diff --git a/src/models/ModelInterface.jl b/src/models/ModelInterface.jl
index d1b2d6e..4e9e0ec 100644
--- a/src/models/ModelInterface.jl
+++ b/src/models/ModelInterface.jl
@@ -387,11 +387,7 @@ function typecheck_instance(
 
   overload_errormsg =
    "the types for this model declaration do not permit Julia overloading to distinguish between GAT overloads"
-
   for (decl, resolver) in theory.resolvers
-    if nameof(decl) ∈ ext_functions
-      continue
-    end
     for (_, x) in allmethods(resolver)
       if getvalue(theory[x]) isa AlgStruct 
         continue 
@@ -455,6 +451,7 @@ function typecheck_instance(
   end
 
   for (sig, (decl, method)) in undefined_signatures
+    nameof(decl) ∈ ext_functions && continue # assume it has been impl'd already
     judgment = getvalue(theory[method])
     if judgment isa AlgTermConstructor
       error("Failed to implement $decl: $(toexpr(sig))")
diff --git a/test/models/ModelInterface.jl b/test/models/ModelInterface.jl
index 5518164..e6fe937 100644
--- a/test/models/ModelInterface.jl
+++ b/test/models/ModelInterface.jl
@@ -170,4 +170,30 @@ end
   munit() = 0
 end
 
+# Test scenario where we @import a method but then extend it
+############################################################
+@theory T1 begin 
+  X::TYPE; 
+  h(a::X)::X 
+end
+
+@theory T2<:T1 begin 
+  Y::TYPE; 
+  h(b::Y)::Y 
+end
+
+@instance T1{Int} begin 
+  h(a::Int) = 1 
+end
+
+@instance T2{Int,Bool} begin 
+  @import X, h
+  h(b::Bool) = false 
+end
+
+@test h(2) == 1
+
+@test h(false) == false
+
+
 end # module