diff --git a/crates/wit-component/tests/shared_instance_type.rs b/crates/wit-component/tests/shared_instance_type.rs
new file mode 100644
index 0000000000..602662a3ee
--- /dev/null
+++ b/crates/wit-component/tests/shared_instance_type.rs
@@ -0,0 +1,102 @@
+use anyhow::{Context, Result, bail};
+use pretty_assertions::assert_eq;
+use wasmparser::{Validator, WasmFeatures};
+use wit_component::{DecodedWasm, WitPrinter, decode};
+use wit_parser::{InterfaceId, Type, TypeDefKind, World, WorldItem, WorldKey};
+
+#[test]
+fn two_implements_imports_share_an_instance_type() -> Result<()> {
+    let wasm = wat::parse_str(
+        r#"
+        (component
+          (type (instance
+            (type (variant (case "other" string)))
+            (export "error" (type (eq 0)))
+            (type (func (result 1)))
+            (export "open" (func (type 2)))
+          ))
+          (import "primary" (implements "a:b/store@0.1.0") (instance (type 0)))
+          (import "backup" (implements "a:b/store@0.1.0") (instance (type 0)))
+        )
+        "#,
+    )?;
+    Validator::new_with_features(WasmFeatures::all())
+        .validate_all(&wasm)
+        .context("validation accepts this component")?;
+
+    let decoded = decode(&wasm).context("decoding the component to WIT")?;
+    let (resolve, world_id) = match decoded {
+        DecodedWasm::Component(resolve, world) => (resolve, world),
+        DecodedWasm::WitPackage(_, _) => bail!("expected a component, decoded a WIT package"),
+    };
+
+    let world = &resolve.worlds[world_id];
+    let primary = interface_import(world, "primary")?;
+    let backup = interface_import(world, "backup")?;
+    assert_eq!(primary, backup, "both imports are the same interface");
+    assert_eq!(world.imports.len(), 2);
+
+    let interface = &resolve.interfaces[primary];
+    assert_eq!(interface.name.as_deref(), Some("store"));
+    let package = &resolve.packages[interface.package.context("interface has a package")?];
+    assert_eq!(package.name.to_string(), "a:b@0.1.0");
+
+    let error_id = *interface
+        .types
+        .get("error")
+        .context("store exports the error type")?;
+    match &resolve.types[error_id].kind {
+        TypeDefKind::Variant(variant) => {
+            assert_eq!(variant.cases.len(), 1);
+            assert_eq!(variant.cases[0].name, "other");
+            assert_eq!(variant.cases[0].ty, Some(Type::String));
+        }
+        other => bail!("error has kind {other:?}, expected a variant"),
+    }
+    let open = interface
+        .functions
+        .get("open")
+        .context("store exports open")?;
+    assert!(open.params.is_empty());
+    assert_eq!(open.result, Some(Type::Id(error_id)));
+
+    let mut printer = WitPrinter::default();
+    let nested = resolve
+        .packages
+        .iter()
+        .map(|(id, _)| id)
+        .filter(|id| *id != resolve.worlds[world_id].package.unwrap())
+        .collect::<Vec<_>>();
+    printer
+        .print(&resolve, resolve.worlds[world_id].package.unwrap(), &nested)
+        .context("printing WIT")?;
+    assert_eq!(
+        printer.output.to_string(),
+        "\
+package root:component;
+
+world root {
+  import primary: a:b/store@0.1.0;
+  import backup: a:b/store@0.1.0;
+}
+package a:b@0.1.0 {
+  interface store {
+    variant error {
+      other(string),
+    }
+
+    open: func() -> error;
+  }
+}
+"
+    );
+    Ok(())
+}
+
+fn interface_import(world: &World, name: &str) -> Result<InterfaceId> {
+    match world.imports.get(&WorldKey::Name(name.to_string())) {
+        Some(WorldItem::Interface { id, .. }) => Ok(*id),
+        Some(_) => bail!("import `{name}` is not an interface"),
+        None => bail!("missing import `{name}`"),
+    }
+}
diff --git a/crates/wit-parser/src/decoding.rs b/crates/wit-parser/src/decoding.rs
index aeddc85eb9..c3d659be98 100644
--- a/crates/wit-parser/src/decoding.rs
+++ b/crates/wit-parser/src/decoding.rs
@@ -2,8 +2,7 @@ use crate::*;
 use alloc::string::{String, ToString};
 use alloc::vec;
 use alloc::vec::Vec;
-use anyhow::Result;
-use anyhow::{Context, anyhow, bail};
+use anyhow::{Context, Result, anyhow, bail, ensure};
 use core::mem;
 use std::io::Read;
 use wasmparser::Chunk;
@@ -796,7 +795,10 @@ impl WitPackageDecoder<'_> {
                                 _ => unreachable!(),
                             }
                             let prev = self.type_map.insert(created, id);
-                            assert!(prev.is_none());
+                            ensure!(
+                                prev.is_none() || prev == Some(id),
+                                "type export `{name}` was already mapped to a different type",
+                            );
                         }
 
                         // If the name is not defined, however, then there's two
@@ -820,7 +822,7 @@ impl WitPackageDecoder<'_> {
                             let prev = self.resolve.interfaces[interface]
                                 .types
                                 .insert(name.to_string(), id);
-                            assert!(prev.is_none());
+                            ensure!(prev.is_none());
                         }
                     }
                 }
@@ -844,7 +846,7 @@ impl WitPackageDecoder<'_> {
                     let prev = self.resolve.interfaces[interface]
                         .functions
                         .insert(name.to_string(), func);
-                    assert!(prev.is_none());
+                    ensure!(prev.is_none());
                 }
 
                 _ => bail!("instance type export `{name}` is not a type"),