Make constraint system fail hard on registering existing variable

compiler/resolution.common/src/org/jetbrains/kotlin/resolve/calls/inference/model/NewConstraintSystemImpl.kt

@@ -115,7 +115,8 @@ class NewConstraintSystemImpl(
         checkState(State.BUILDING, State.COMPLETION, State.TRANSACTION)
 
         transactionRegisterVariable(variable)
-        storage.allTypeVariables[variable.freshTypeConstructor()] = variable
+        storage.allTypeVariables.put(variable.freshTypeConstructor(), variable)
+            ?.let { error("Type variable already registered: old: $it, new: $variable") }
         notProperTypesCache.clear()
         storage.notFixedTypeVariables[variable.freshTypeConstructor()] = MutableVariableWithConstraints(this, variable)
     }