When your significant other looks like they could be your sibling.
BUDDY: Did you see Pat's sibnificant other?
BRO: Yea, dude, name's Kelly, and I know what you mean. I agree 100%.
BUDDY: Maybe they should have a heart to heart with their parents, or get some DNA testing.
BRO: Really, that's some freaky shit. I'm not sure that that's just a coincidence, I mean, they just look way too much alike!