It is possible to deduplicate contacts entered in a Webform?
They are merged, based on the "Unsupervised" deduplication rules: civicrm/contact/deduperules?reset=1

By default the unsupervised rules for individuals are based on email-only.
So if someone fills in a Webform with an email that already exists in civicrm then those data will be merged.

You can change these rules: http://book.civicrm.org/user/common-workflows/deduping-and-merging/