Scenario:
1. mkdir test
2. cd test
3. darcs init
4. touch file
5. darcs add file
6. darcs record...
7. darcs tag v1.0
8. more changes and records
9. darcs tag v1.0
10. darcs list tags

Problems:
As a new user I don't know which tag is which and having the duplicates listed is no benefit for me.
If the tagging was a mistake (a user created a duplicate tag by accident) I have no information from this 
command that would help in obliterating the buggy tag or allowing me to go back to a specific tag from the 
list.
If it's intended to create duplicates in order to have a 'moving' tag then listing all the duplicates does 
not provided any information needed by the developer as he is always working with the latest tag.

Expected:
I expect one of two possible solutions:
1. Don't display duplicates by default or just display a count of duplicates
2. Display an unique identifier (hash) of the tag next to it's name if duplicates exist.

Makes no sense IMO. If duplicate tags exist (bad enough) I would
not want them to be hidden.