delete duplicate aliases created by itunes when adding songs stored in my external hard drive
Try Dupin or Dupin Lite. Dupin is $15 from the developer's site, and Dupin Lite is $8 from the Mac App Store. I recently purchased the full version and used it to delete duplicates from a 57,000 song library. It did a very good job. It's filtering mechanism is highly customizable. It is not super fast, but it ran through my library in less than a day.

To specifically deal with your issue, you can set the rules to keep the file with the shorter filename. This will get rid of the commonly found "whatever the song name 1.mp3" file, while keeping the "whatever the song name.mp3" version.

Also, dougscripts.com should be your first stop for any iTunes maintenance scripts. I have found the following scripts especially useful in cleaning up large iTunes Libraries:
Artist - Name Corrector
Super Remove Dead Tracks
Search-Replace Tag Text v2.0
Track Parser
Remove n Characters From Front or Back