When you unshelve a shelveset, the following dialog box may pop up:
The item exists in your pending changes window. It might be caused by that the "Preserve Pending Changes Locally" option was checked when you performed Shelve.
Undo the corresponding pending changes. Or, if you do not need to further edit the files in the shelveset on performing Shelve, please uncheck the "Preserve Pending Changes Locally" option.