feat(store): warn when same action is registered (#1801)

Closes #1758

modules/store/spec/reducer_creator.spec.ts

@@ -1,3 +1,4 @@
+import * as ngCore from '@angular/core';
 import { on, createReducer, createAction, props, union } from '@ngrx/store';
 import { expecter } from 'ts-snippet';
 
@@ -86,6 +87,59 @@ import {on} from './modules/store/src/reducer_creator';
         state = fooBarReducer(state, bar({ bar: 54 }));
         expect(state).toEqual(['[foobar] FOO', '[foobar] BAR']);
       });
+
+      describe('warning message when the same action type gets registered', () => {
+        it('should not warn when not violated', () => {
+          const spy = spyOn(console, 'warn');
+
+          const fooBarReducer = createReducer(
+            [],
+            on(foo, state => state),
+            on(bar, state => state)
+          );
+
+          expect(spy).not.toHaveBeenCalled();
+        });
+
+        it('should warn in dev mode', () => {
+          const spy = spyOn(console, 'warn');
+
+          const fooBarReducer = createReducer(
+            [],
+            on(foo, state => state),
+            on(bar, state => state),
+            on(foo, state => state)
+          );
+
+          expect(spy).toHaveBeenCalledTimes(1);
+        });
+
+        it('should warn in dev mode with multiple actions', () => {
+          const spy = spyOn(console, 'warn');
+
+          const fooBarReducer = createReducer(
+            [],
+            on(foo, foo, state => state),
+            on(bar, state => state)
+          );
+
+          expect(spy).toHaveBeenCalledTimes(1);
+        });
+
+        it('should not warn in prod mode', () => {
+          spyOn(ngCore, 'isDevMode').and.returnValue(false);
+          const spy = spyOn(console, 'warn');
+
+          const fooBarReducer = createReducer(
+            [],
+            on(foo, state => state),
+            on(bar, state => state),
+            on(foo, state => state)
+          );
+
+          expect(spy).not.toHaveBeenCalled();
+        });
+      });
     });
   });
 });

modules/store/src/reducer_creator.ts

@@ -1,3 +1,4 @@
+import { isDevMode } from '@angular/core';
 import { ActionCreator, ActionReducer, ActionType, Action } from './models';
 
 // Return type of the `on` fn.
@@ -51,8 +52,15 @@ export function createReducer<S>(
   ...ons: On<S>[]
 ): ActionReducer<S> {
   const map = new Map<string, ActionReducer<S>>();
+  const devMode = isDevMode();
+
   for (let on of ons) {
     for (let type of on.types) {
+      if (devMode && map.has(type)) {
+        console.warn(
+          `@ngrx/store: The provided action type '${type}' is already provided.`
+        );
+      }
       map.set(type, on.reducer);
     }
   }