Songs in your iTunes library can come from different sources (not just from a single online source), so iTunes does not want to assume a "duplicated" song is really a duplicate song. For example, you may have the same song from two different albums, the original album and a separate "Best of" album. It may be the same song, but you may want both to keep the two albums complete. Or you may have two different recordings of the same song from the same artist, a "studio" recording and a "live" recording. So, there's no (potentially dangerous) command to "delete duplicates" in iTunes.
There's a command to show a list of duplicate songs, from iTunes menu bar
File -> Library -> Show Duplicate Items
If you press the Option key while using command, it becomes
File -> Library -> Show Exact Duplicate Items
That gives you a better list of songs iTunes considers to be "exactly" the same. If you don't want to review the items identified carefully, you can select "every other" item and delete them all at the same time.
NOTE: To select multiple (non-consecutive) items at the same time on an iTunes list, press and hold down the Option and Command keys, then click items on list. All clicked items are selected at the same time. Then, press Delete on keyboard.