Yes! I finally got an answer. After hours of trying everything posted plus some, I did something really simple and it worked. I disconnected my external drive (which has a back-up of my Itunes Music on it) and then turned my computer off. I turned my computer back on, waited for the reboot, and confirmed that the external device did not appear under devices. I then opened Itunes and highlighted all my music files in the music library, and then hit delete. The banner came up asking if I was sure I wanted to delete and instead I hit cancel which unhighlighted the list, AND removed all the exclamation marks. By disconnecting the external drive, Itunes was forced to look for the music on the Mac hardrive where it all still existed after I moved a copy of the library from the external to the mac when I first started having issues with the exclamation marks, and all the duplicates are gone too.
My entire Itunes Music Library is intact with everything still there, uploaded cd's, downloaded music, etc., and it all plays fine. For me and my system, disconnecting the external drive forced Itunes to look for the library on my mac hardrive, which it did, and it put everything back in order. I have a back up of my library on the external drive and maybe that confused Itunes regarding location when I first moved the music library there when I was running out of space on my mac hardrive. I did not change the location in Itunes however and so the exclamation marks showed up at some point when I next had a look at the list. I moved the library back onto my mac hardrive with a copy on the external and when I asked for the location information, it had more than one place to identify and it became confusing. Unplugging the external was they key. Don't know what will happen when I attach my Terrabyte drive back to the Mac, but I think it will be nothing. Now that it has identified the location, the list is good. I'm not a techie, just tenacious. I have no idea if this will work for anyone else, but here you have it. Thanks for asking, and I hope it works for you.