I have on occasion found myself looking too long and / or too often at a woman, because they genuinely reminded me of someone else, and it was hard not to.
I have twice felt the need to go over and explain, though I'm sure they just thought it was a line.
Any advice on if it was at the right thing to do ? In the more recent case, the doppelganger was a musical celebrity, so I could have quickly pulled up a photo of them on my phone.