I know that I should say: It sounds strange AND It sounds strangely familiar.
Why in the first instance it is OK to write "strange" and in the second one "strangely"?
In the first one it's an adjective. It sounds strange, your strange story.
In the second, strangely is an adverb modifying familiar. Your familiar story sounds strangely familiar. Compare: It looks terrible. It looks terribly old.