To see how many times a clip was duplicated in a sequence you can highlight the sequence in the Browser and click Apple 0 (Zero) to open your Sequence Settings. In the timeline options tab you can turn on Show Duplicate Frames which will give you a color bar under your duplicated clips in the timeline. This only tells you what is exactly duplicated frame for frame. To adjust those settings you need to go to User Preferences and adjust Dupe Detection Handle Size and Threshold under the Editing tab.
To see what has been used in your sequence you can highlight the sequence in the bin, click Apple F, set the For section to Used Media, click In Selected Sequences to on, and make sure Replace Find Results is selected. This will give you a list of all used media in your project as long as the media information is located somewhere in the Browser. If you grab media from your hard drive and place it directly on your Sequence it may not show up in the new bin.
You can then color Label all the clips in the Found Bin and that will also change the color of the clips in your original bins.
good luck
Victor