Creating manuals with the same title

[chrisroos]

To replicate:

1. Create a new manual
2. Observe that the manual has been created as expected
3. Create a new manual using the same title as the manual created in step 1
4. Observe that an exception is raised
5. Observe that there are now 3 manuals with the same title (instead of the 2 that you might expect)

It's not obvious what the expected behaviour is in this situation but an exception being raised definitely isn't ideal.

This might be related to issue #1114.

[chrisroos]

PR #1148 goes some way toward this in that it should avoid us creating multiple manuals in our local database. The exception from the Publishing API will still be raised.