Import fails on duplicate tags / add_date ignored

1) (c1e07f0) When a link in an imported file has duplicate tags (e.g. tag="read,web,web"), the import failed with an error message "SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '441-web' for key 'bookmark_tag'" in the log file. The commit ignores already existing tags in the database.

2) (abe6bdb) The add_date attributed in the imported files were ignored. Like that, the added-date was the time of import, not the time when the link was originally added.

The html-file I used was exported from Delicious. Duplicate tags are basically an error on their side, but owncloud should be tolerant for that.