Consolidation of duplicated tags

### Expected
I have duplicated tags (two tags with the same name).
I delete one of the two tags.
The notes shall remain in the remaining (single) tag.

### Observed
The notes loose the tag at all.
(I have many notes.)

### Reproduced
see above

### Remark
The new appearance of duplicated tags is avoided with Simplenote versions starting with 2.x

### Where did you see the bug

- OS: Windows 2010
- Simplenote app version: 2.4.0

Start by reproducing the duplicate-tag deletion scenario described for Simplenote 2.4.0 on Windows, using notes that share the duplicated tag. Trace the tag deletion and note-association flow to determine where the remaining tag should retain those notes. Done means deleting one duplicate leaves every affected note associated with the single remaining tag, with regression coverage for the scenario.

Written by the indexing model from the issue text.