Remove duplicate tags on paste

[stehlikio]

I recently ran into this and saw issue #795. It's a decent workaround, but a more permanent fix might be helpful.

It looks like when adding all of the tags from the paste, since everything is running async, the tags aren't present in the items array at the time of the duplicate check. We can get in front of this issue by removing duplicates from the split text before we try to insert duplicate values and have to check later down the road.

[coveralls]

Coverage increased (+0.0004%) to 99.801% when pulling 2d5e7c8 on stehlikio:fix-paste-dups into 87d0e6b on mbenford:master.

[samfitz66]

Any chance this could be released? :)

[realph]

+1 to have this merged in. @mbenford any plans?