fix regression with imported distinct types

compiler/mir/mirtypes.nim

@@ -1085,11 +1085,13 @@ proc handleImported(env: var TypeEnv, t: PType): TypeId =
   if t.sym != nil and sfImportc in t.sym.flags:
     # an imported type. It's wrapped in a ``tkImported``, referencing the
     # underlying type
-    let base = typeSymToMir(env):
+    let base =
       if t.kind in Skip:
-        t.lastSon.skipIrrelevant()
+        env.add t.lastSon.skipIrrelevant()
       else:
-        t
+        # create a type symbol without going through the cache or ``.importc``
+        # handling
+        typeSymToMir(env, t)
 
     discard getSize(env.config, t) # compute the sizes, alignments, and offsets
 

tests/ccgbugs/timport_distinct.nim

@@ -0,0 +1,15 @@
+discard """
+  description: '''
+    Regression test for importing ``distinct`` types resulting in duplicate C
+    type definitions
+  '''
+"""
+
+type
+  Object = object
+  Imported {.importc: "int", nodecl.} = distinct Object
+
+var x: Object
+var y: Imported
+
+# two type definitions for `Object` were emitted