If an app is installed via market, market update, or download from browser and the user launches the app directly from the installer (ie: when the installer completes it offers the user the option to launch the app now) it is launched in such a way that the OS gets confused.

If, after the app is launched, the user presses the HOME key to return to the home screen and then tries to return to the app by selecting it from the list of applications (or by putting a shortcut on the home screen and then selecting the shortcut), the OS launches the root activity of the app AGAIN, bringing the existing task to the foreground and placing ANOTHER instance of the root activity on top of the existing activity stack.

This is fundamentally broken as this is very different from the behaviour if the user initially launched the app by selecting it from the list of available applications. In that case, when the user presses the HOME key and then tries to return to the application by selecting it from the list of applications (or selecting the shortcut),
the OS simply brings the existing task to the foreground. It does NOT create another instance of the root activity.

This behaviour is extremely difficult to debug and has certainly caused countless hours of head-scratching by developers.

This issue was originally reported 3 years ago (by someone else) as issue 36907463 but has not received any attention probably because it references "eclipse" in the summary line. This issue has nothing to do with eclipse. It just also happens to occur if you initially launch the app via eclipse or another IDE.

It isn't a problem only for developers. It occurs on real devices running all versions of Android OS, even Android 4.0! We've seen this on HTC Desire running 2.2, HTC Desire S running 2.3, HTC sensation running 4.0 and others.

We've created a sample application that can be used to demonstrate the behaviour. Here is an explanation and step-by-step instructions:

ActivityA is the starting activity. It has a button that will launch
ActivityB and it finishes itself after that launch. This means that if you press the BACK key in ActivityB the home screen will be displayed.
Both activities have a TextView that indicates whether the app is before or after the update of the application.

There are 2 APKs provided: "before.apk" and "after.apk". They both contain exactly the same code, only the version number in the manifest has been changed (this is so we can simulate the "update" from the market).

In order to explain it more clearly, read the following step-by-step instructions. The source code of the sample app, the apk files, the logs and some screenshots can be found in the attached zip file.

00. Preconditions:
Have the "before.apk" installed on your device and the "after.apk" present somewhere on the device's file system (SD-card or whatever).

01. Start the app by using the respective icon.

02. ActivityA is displayed. Press button "launch AcitivtyB" in ActivityA.

03. ActivityB is displayed. Press HOME key. Open any file manager application and click on the "after.apk" and install it. This means
that "before.apk" will be replaced with "after.apk".

04. Once the installation is complete, press the "Open" button in the installer (see 03.png).

05. ActivityA is displayed. Press button "launch ActivityB".

06. ActivityB is displayed. Press HOME key in ActivityB.

07. Launch the application again by using the respective icon.

08. ActivityA is displayed. Notice that in the dumpsys (07.txt) you can see that both ActivityA and ActivityB are active but this shouldn't be the case because ActivityB should only be active after ActivityA has been finished.

Press button "launch ActivityB".

09. ActivityB is displayed. Notice that in the dumpsys (08.txt) you can see that two instances of ActivityB are active.

Press BACK key in ActivityB.

09. ActivityB is displayed. Notice that in the dumpsys (09.txt) you can see that one instance of ActivityB is active.

Press BACK key in ActivityB.

10. The home screen is displayed. Notice that in the dumpsys (10.txt)
you can see that no instance of ActivityA or ActivityB is active anymore.