angular - Page is part of the declarations of 2 modules: Error in ionic build prod

Question

When I run npm run ionic:build I am able to build successfully. But When I run npm run ionic:build --prod I am getting following error message.

Error: Type PatientDetailPage in /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.ts is 
            part of the declarations of 2 modules: AppModule in /home/shiva/Ionic/af2-lists/src/app/app.module.ts and 
            PatientDetailPageModule in /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.module.ts! 
            Please consider moving PatientDetailPage in 
            /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.ts to a higher module that imports 
            AppModule in /home/shiva/Ionic/af2-lists/src/app/app.module.ts and PatientDetailPageModule in 
            /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.module.ts. You can also create a new 
            NgModule that exports and includes PatientDetailPage in 
            /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.ts then import that NgModule in 
            AppModule in /home/shiva/Ionic/af2-lists/src/app/app.module.ts and PatientDetailPageModule in 
            /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.module.ts. 
Error: Type PatientDetailPage in /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.ts is part of the declarations of 2 modules: AppModule in /home/shiva/Ionic/af2-lists/src/app/app.module.ts and PatientDetailPageModule in /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.module.ts! Please consider moving PatientDetailPage in /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.ts to a higher module that imports AppModule in /home/shiva/Ionic/af2-lists/src/app/app.module.ts and PatientDetailPageModule in /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.module.ts. You can also create a new NgModule that exports and includes PatientDetailPage in /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.ts then import that NgModule in AppModule in /home/shiva/Ionic/af2-lists/src/app/app.module.ts and PatientDetailPageModule in /home/shiva/Ionic/af2-lists/src/pages/patient-detail/patient-detail.module.ts.
    at syntaxError (/home/shiva/Ionic/af2-lists/node_modules/@angular/compiler/bundles/compiler.umd.js:1550:34)
    at CompileMetadataResolver._addTypeToModule (/home/shiva/Ionic/af2-lists/node_modules/@angular/compiler/bundles/compiler.umd.js:14655:31)
    at /home/shiva/Ionic/af2-lists/node_modules/@angular/compiler/bundles/compiler.umd.js:14543:27
    at Array.forEach (native)
    at CompileMetadataResolver.getNgModuleMetadata (/home/shiva/Ionic/af2-lists/node_modules/@angular/compiler/bundles/compiler.umd.js:14534:54)
    at addNgModule (/home/shiva/Ionic/af2-lists/node_modules/@angular/compiler/bundles/compiler.umd.js:23050:58)
    at /home/shiva/Ionic/af2-lists/node_modules/@angular/compiler/bundles/compiler.umd.js:23061:14
    at Array.forEach (native)
    at _createNgModules (/home/shiva/Ionic/af2-lists/node_modules/@angular/compiler/bundles/compiler.umd.js:23060:26)
    at analyzeNgModules (/home/shiva/Ionic/af2-lists/node_modules/@angular/compiler/bundles/compiler.umd.js:22935:14)

I understand that I am loading same modules multiple times, But could not understand how to remove them.

my app.module.ts

@NgModule({
  imports: [
    BrowserModule,
    IonicModule.forRoot(MyApp),
    AngularFireDatabaseModule,
    AngularFireModule.initializeApp(firebaseConfig),
    AngularFireOfflineModule,
    IonicStorageModule.forRoot()
   // PatientDetailPage

  ],
  declarations: [
    MyApp,
    HomePage,
    PatientDetailPage
  ],
  entryComponents: [
    MyApp,
    HomePage,
    PatientDetailPage
  ],
  providers: [
    StatusBar,
    SplashScreen,
    //AngularFireModule,
    //Storage,
    {provide: ErrorHandler, useClass: IonicErrorHandler}
  ],
   bootstrap: [IonicApp],
})

patient-detail.module.ts is

import { NgModule } from '@angular/core';
import { IonicPageModule } from 'ionic-angular';
import { PatientDetailPage } from './patient-detail';

@NgModule({
  declarations: [
    PatientDetailPage,
  ],
  imports: [
    IonicPageModule.forChild(PatientDetailPage),
  ]

})
export class PatientDetailPageModule {

}

Answer 1

This is a basic error of angular. You can see the issue here. So the answer that is accepted till now is use share module. You can do like this:
- Create a share.module.ts
- Import, declaration and export your component in this share.module.ts

import { NgModule }       from '@angular/core';
import {SharedComponentA} from "./SharedComponentA";
import {SharedComponentB} from "./SharedComponentB";

@NgModule({
    imports: [
    ],
    declarations: [
      SharedComponentA,
      SharedComponentB

    ],
    providers: [
    ],
    exports: [
      SharedComponentA,
      SharedComponentB
    ]
})
export class SharedModule {}

- If you want to use your component in a Ionic Page(lazy load page), import share module in the module of that Ionic page:

import {SharedModule } from './SharedModule';
@NgModule({
    imports: [
        SharedModule    
        //..
    ],
    //..
})

- If you want to use your component in other component or no lazy load page, import share module in your app.module.ts like above.
See more about ngModule