diff --git a/Cargo.lock b/Cargo.lock
index 61e1c4a..1a18f3b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -26,6 +26,12 @@ version = "0.7.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "96d30a06541fbafbc7f82ed10c06164cfbd2c401138f6addd8404629c4b16711"
 
+[[package]]
+name = "autocfg"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d468802bab17cbc0cc575e9b053f41e72aa36bfa6b7f55e3529ffa43161b97fa"
+
 [[package]]
 name = "base64ct"
 version = "1.6.0"
@@ -42,6 +48,21 @@ dependencies = [
  "rand",
 ]
 
+[[package]]
+name = "bit-set"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0700ddab506f33b20a03b13996eccd309a48e5ff77d0d95926aa0210fb4e95f1"
+dependencies = [
+ "bit-vec",
+]
+
+[[package]]
+name = "bit-vec"
+version = "0.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "349f9b6a179ed607305526ca489b34ad0a41aed5f7980fa90eb03160b69598fb"
+
 [[package]]
 name = "bitflags"
 version = "1.3.2"
@@ -164,6 +185,12 @@ version = "2.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "25cbce373ec4653f1a01a31e8a5e5ec0c622dc27ff9c4e6606eefef5cbbed4a5"
 
+[[package]]
+name = "fnv"
+version = "1.0.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"
+
 [[package]]
 name = "generic-array"
 version = "0.14.7"
@@ -198,12 +225,24 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "lazy_static"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
+
 [[package]]
 name = "libc"
 version = "0.2.150"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "89d92a4743f9a61002fae18374ed11e7973f530cb3a3255fb354818118b2203c"
 
+[[package]]
+name = "libm"
+version = "0.2.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4ec2a862134d2a7d32d7983ddcdd1c4923530833c9f2ea1a44fc5fa473989058"
+
 [[package]]
 name = "linux-raw-sys"
 version = "0.4.12"
@@ -215,11 +254,22 @@ name = "mast"
 version = "0.1.0"
 dependencies = [
  "blake3",
+ "proptest",
  "redb",
  "tempfile",
  "varu64",
 ]
 
+[[package]]
+name = "num-traits"
+version = "0.2.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "39e3200413f237f41ab11ad6d161bc7239c84dcb631773ccd7de3dfe4b5c267c"
+dependencies = [
+ "autocfg",
+ "libm",
+]
+
 [[package]]
 name = "password-hash"
 version = "0.5.0"
@@ -246,6 +296,32 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "proptest"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "31b476131c3c86cb68032fdc5cb6d5a1045e3e42d96b69fa599fd77701e1f5bf"
+dependencies = [
+ "bit-set",
+ "bit-vec",
+ "bitflags 2.4.1",
+ "lazy_static",
+ "num-traits",
+ "rand",
+ "rand_chacha",
+ "rand_xorshift",
+ "regex-syntax",
+ "rusty-fork",
+ "tempfile",
+ "unarray",
+]
+
+[[package]]
+name = "quick-error"
+version = "1.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a1d01941d82fa2ab50be1e79e6714289dd7cde78eba4c074bc5a4374f650dfe0"
+
 [[package]]
 name = "quote"
 version = "1.0.33"
@@ -285,6 +361,15 @@ dependencies = [
  "getrandom",
 ]
 
+[[package]]
+name = "rand_xorshift"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d25bf25ec5ae4a3f1b92f929810509a2f53d7dca2f50b794ff57e3face536c8f"
+dependencies = [
+ "rand_core",
+]
+
 [[package]]
 name = "redb"
 version = "1.4.0"
@@ -303,6 +388,12 @@ dependencies = [
  "bitflags 1.3.2",
 ]
 
+[[package]]
+name = "regex-syntax"
+version = "0.8.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c08c74e62047bb2de4ff487b251e4a92e24f48745648451635cec7d591162d9f"
+
 [[package]]
 name = "rustix"
 version = "0.38.28"
@@ -316,6 +407,18 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "rusty-fork"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb3dcc6e454c328bb824492db107ab7c0ae8fcffe4ad210136ef014458c1bc4f"
+dependencies = [
+ "fnv",
+ "quick-error",
+ "tempfile",
+ "wait-timeout",
+]
+
 [[package]]
 name = "snafu"
 version = "0.6.10"
@@ -404,6 +507,12 @@ version = "1.17.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "42ff0bf0c66b8238c6f3b578df37d0b7848e55df8577b3f74f92a69acceeb825"
 
+[[package]]
+name = "unarray"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eaea85b334db583fe3274d12b4cd1880032beab409c0d774be044d4480ab9a94"
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.12"
@@ -425,6 +534,15 @@ version = "0.9.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "49874b5167b65d7193b8aba1567f5c7d93d001cafc34600cee003eda787e483f"
 
+[[package]]
+name = "wait-timeout"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9f200f5b12eb75f8c1ed65abd4b2db8a6e1b138a20de009dacee265a2498f3f6"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "wasi"
 version = "0.11.0+wasi-snapshot-preview1"
diff --git a/mast/Cargo.toml b/mast/Cargo.toml
index 06ef672..17b4ff9 100644
--- a/mast/Cargo.toml
+++ b/mast/Cargo.toml
@@ -11,4 +11,5 @@ redb = "1.4.0"
 varu64 = "0.7.0"
 
 [dev-dependencies]
+proptest = "1.4.0"
 tempfile = "3.8.1"
diff --git a/mast/proptest-regressions/operations/insert.txt b/mast/proptest-regressions/operations/insert.txt
new file mode 100644
index 0000000..93591fb
--- /dev/null
+++ b/mast/proptest-regressions/operations/insert.txt
@@ -0,0 +1,7 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc 7912e0f85e7a1a9892a9c85944f3392f6e1fe1a33d2ea00b3809400d75166743 # shrinks to random_entries = [([135], [150]), ([25], [0]), ([129], [0]), ([135], [150])]
diff --git a/mast/src/operations/insert.rs b/mast/src/operations/insert.rs
index a6cb1ce..03eeb39 100644
--- a/mast/src/operations/insert.rs
+++ b/mast/src/operations/insert.rs
@@ -116,7 +116,12 @@ pub fn insert(
     if let Some(mut existing) = path.existing {
         if existing.value() == value {
             // There is really nothing to update. Skip traversing upwards.
-            return path.upper_path.pop().map(|(n, _)| n).unwrap_or(existing);
+
+            return path
+                .upper_path
+                .first()
+                .map(|(n, _)| n.clone())
+                .unwrap_or(existing);
         }
 
         // Decrement the old version.
@@ -222,6 +227,35 @@ fn binary_search_path(
 #[cfg(test)]
 mod test {
     use crate::test::{test_operations, Entry};
+    use proptest::prelude::*;
+
+    proptest! {
+        #[test]
+        /// Test that upserting an entry with the same key in different tree shapes results in the
+        /// expected structure
+        fn test_upsert(random_entries in prop::collection::vec(
+            (prop::collection::vec(any::<u8>(), 1), prop::collection::vec(any::<u8>(), 1)),
+            1..10,
+        )) {
+            let operations = random_entries.into_iter().map(|(key, value)| {
+                Entry::insert(&key, &value)
+            }).collect::<Vec<_>>();
+
+            test_operations(&operations, None);
+        }
+
+        #[test]
+        fn test_general_insertiong(random_entries in prop::collection::vec(
+            (prop::collection::vec(any::<u8>(), 32), prop::collection::vec(any::<u8>(), 32)),
+            1..50,
+        )) {
+            let operations = random_entries.into_iter().map(|(key, value)| {
+                Entry::insert(&key, &value)
+            }).collect::<Vec<_>>();
+
+            test_operations(&operations, None);
+        }
+    }
 
     #[test]
     fn insert_single_entry() {
diff --git a/mast/src/test.rs b/mast/src/test.rs
index 8d2a2be..5c0efbf 100644
--- a/mast/src/test.rs
+++ b/mast/src/test.rs
@@ -162,12 +162,12 @@ fn into_mermaid_graph(treap: &HashTreap) -> String {
 }
 
 fn build_graph_string(treap: &HashTreap, node: &Node, graph: &mut String) {
-    let key = bytes_to_string(node.key());
+    let key = format_key(node.key());
     let node_label = format!("{}(({}))", node.hash(), key);
 
     // graph.push_str(&format!("## START node {}\n", node_label));
     if let Some(child) = treap.get_node(node.left()) {
-        let key = bytes_to_string(child.key());
+        let key = format_key(child.key());
         let child_label = format!("{}(({}))", child.hash(), key);
 
         graph.push_str(&format!("    {} --l--> {};\n", node_label, child_label));
@@ -178,7 +178,7 @@ fn build_graph_string(treap: &HashTreap, node: &Node, graph: &mut String) {
     }
 
     if let Some(child) = treap.get_node(node.right()) {
-        let key = bytes_to_string(child.key());
+        let key = format_key(child.key());
         let child_label = format!("{}(({}))", child.hash(), key);
 
         graph.push_str(&format!("    {} --r--> {};\n", node_label, child_label));
@@ -189,6 +189,6 @@ fn build_graph_string(treap: &HashTreap, node: &Node, graph: &mut String) {
     }
 }
 
-fn bytes_to_string(byte: &[u8]) -> String {
-    String::from_utf8(byte.to_vec()).expect("Invalid utf8 key in test with mermaig graph")
+fn format_key(bytes: &[u8]) -> String {
+    format!("\"{:?}\"", bytes)
 }