Someone by mistake took my Talis and left over his Talis a few weeks ago and it has no סימן the question is can I use it and can I make a ברכה on it?
This is a very common (and unfortunate occurrence), that things get swapped in shul etc. The Shulchan Aruch CH:M 136-2 talks about you case, that if one’s item got switched at a wedding etc. that he has to return it and he should not use the other person’s item. Try putting up a sign that everyone should check his tallis, because your tallis was switch a few weeks ago …