Cut-and-paste between documents throwing a weird pop-up about "styles

I archive customer designs by copying-and-pasting from the design document into a month-specific archive file. Been doing this for 15 years. Suddenly when copying from one specific design document, I'm getting the following pop-up when I try pasting it and have no clue what it means by styles

Some imported styles have names that already exist in the document. Merge styles?
Choosing No will automatically rename them.