Wait until somebody comes in who knows what they are talking about, but it looks to me that it just has copper platting issues.
VarietyVista show no doubled dies for 1990D cents, However, from the small images it appears there is a lamination issue above IGWT. I am sure more seasoned eyes can give you a better answer
No, the copper plating has "pulled-away" from the zinc core during striking. This is fairly common but you have a nice specimen of that characteristic.
No. Go on to the internet and search for Wexler, CONECA, Error Ref.com., and doubled die coins. There will be lots of information. Authentic doubled die coins are very scarce. However, coins that look like doubled dies are very common. I've probably have a dozen in my spare change jar if I bothered to look.
Not double.. Doubled Die Quote - "Note that the proper terminology for this occurrence includes the letter 'd' at the end of the first word, hence "doubled die". The term "double die" without the first word ending in 'd' is not proper numismatic terminology." Closed quote https://en.m.wikipedia.org/wiki/Doubled_die peace