Many shuls in the U.S. start maariv around eight minutes before Shabbat is over, so that by the end it will be time for maariv. Doesn’t this create a problem with regard to keriat shema, as keriat shema is read before tzes ha-kokhavim (by a few minutes)? No one has ever assumed that you need to repeat shema again, but why not?
You are bringing up a good point. If indeed the minyan is saying kriyas shema before it is actually nightfall, then everyone has to remember to say kriyas shema again. If people will listen to you, it is a good thing to make an announcement about it.