Same Activity called twice... Issue with Multiple AsyncTasks?

I have three simultaneous instances of an AsyncTask for download three files. When two particular ones finish, at the end of onPostExecute() I check a flag set by each, and if both are true, I call startActivity() for the next Activity.

I am currently seeing the activity called twice, or something that resembles this type开发者_开发百科 of behavior. Since the screen does that 'swipe left' kind of transition to the next activity, it sometimes does it twice (and when I hit back, it goes back to the same activity). It's obvious two versions of the activity that SHOULD only get called once are being put on the Activity stack.

Could this be from both onPostExecute()s executing simultaneously and both checking the flags each other set at the exact same time? This seems extremely unlikely since two processes would have to be running line-by-line in parallel...

*****EDIT*** A lot removed from this question since I was way off in what I thought was wrong. Nonetheless I found the answer here quite useful, so I have edited the question to reflect the useful parts.

The only way I can find that this is possible is if both AsyncTasks' onPostExecute() executed SO simultaneously that they were virtually running the same lines at the same time, since I set the 'itemXdownloaded' flag to true right before I check for both and call startActivity().

Since they are both called on the main application thread, that's not possible, unless you're doing something really strange.

I would introduce some Log calls to ensure that you are not misreading the symptoms.

Beyond that, it is difficult to see any problems from your pseudocode, unless there's a possibility of other downloadID values beyond the three shown. For example, if there is a DL4, and DL4 completed after DL1 and DL2, DL4 would trigger your activity.