It's possible to assign links from one issue to another. It might not always be clear what they mean however. For example, one might wonder what is the difference between "blocks" and "precedes".
The following page on the Redmine FAQ explains the different dependencies rather well:
Basically, "A blocks B" means "the end of A needs to happen before the end of B".
"A precedes B" means "the end of A needs to happen before the beginning of B", with an optional delay between the two.
How to deal with duplicate bugs
If a bug is created, but an issue already exists in the tracker about the exact same problem, the new report should be marked as a duplicate of the first one.
The "master" bug should ideally be the earliest one that was reported (so the one will the smallest issue #). This one will link to each of its potential duplicates.
To mark a bug as a duplicate, close it with status Invalid, then add a link "Duplicates" to the earlier bug. (NOT "Duplicated by"!) Make sure the link is in the right direction, or you might inadvertently close the master bug without wanting to!
For example, if issue 150 has already been reported into issue 85, then 150 must duplicate 80. On 80's page it should say "duplicated by 150". Issue 150 should be closed as Invalid, and all progress/discussion should happen in issue 80.