Duplicate entry errors when changing paths from one that has already existed before

1) Disable pathauto (Never seen this with pathauto enabled, so it may be bypassing the path_redirect_nodeapi logic that causes this, or just wasn't unlucky before.)
2) Create a node with path 'one'
3) Edit node to path 'two'
4) Edit node to path 'one'
5) Edit node to path 'three'
=> Duplicate entry for path one, as it successfully saved a redirect for 'one' after step 3, then tries and resaves it it in 5 without checking if it exists.

Solution: see if entry already exists before saving.