fix(#1558): Discrepancy in addUsedSchema between v6 and v8

lib/compile/resolve.ts

@@ -102,7 +102,7 @@
   traverse(schema, {allKeys: true}, (sch, jsonPtr, _, parentJsonPtr) => {
     if (parentJsonPtr === undefined) return
     const fullPath = pathPrefix + jsonPtr
     let baseId = baseIds[parentJsonPtr]
     if (typeof sch[schemaId] == "string") baseId = addRef.call(this, sch[schemaId])
     addAnchor.call(this, sch.$anchor)
     addAnchor.call(this, sch.$dynamicAnchor)
@@ -112,6 +112,7 @@
       // eslint-disable-next-line @typescript-eslint/unbound-method
       const _resolve = this.opts.uriResolver.resolve
       ref = normalizeId(baseId ? _resolve(baseId, ref) : ref)
+      if (this.opts.addUsedSchema === false) return ref // skip adding schema reference
       if (schemaRefs.has(ref)) throw ambiguos(ref)
       schemaRefs.add(ref)
       let schOrRef = this.refs[ref]

spec/resolve.spec.ts

@@ -134,6 +134,37 @@ uriResolvers.forEach((resolver) => {
             }, /resolves to more than one schema/)
           })
         })
+        it("should not throw if the same id resolves to two different schemas when addUsedSchema is false", () => {
+          const ajvInstance = new _Ajv({
+            allErrors: true,
+            verbose: true,
+            inlineRefs: false,
+            allowUnionTypes: true,
+            uriResolver: resolver,
+            addUsedSchema: false,
+          })
+
+          ajvInstance.compile({
+            $id: "http://example.com/1.json",
+            type: "integer",
+          })
+
+          let errorOccurred = false
+          try {
+            ajvInstance.compile({
+              type: "object",
+              additionalProperties: {
+                $id: "http://example.com/1.json",
+                type: "string",
+              },
+            })
+          } catch (error) {
+            errorOccurred = true
+          }
+
+          // Assert that no error occurred
+          errorOccurred.should.equal(false)
+        })
 
         it("should resolve ids defined as urn's (issue #423)", () => {
           const schema = {