My condolences if you're reading this, because it means that you're being tortured by an extremely useless and vague error message. Let's see if we can get through this without tearing out too much hair. :-)
Deployment Error or DML Error?
If you Google "duplicate value found: <unknown>...", the results you get will cover a wide variety of root causes. The first step is to figure out which scenario is appropriate to you.
If you're getting this error from a DML operation, you're in luck. The possible solutions seem to be more straightforward. Sometimes, there might even only be one "<unknown>" instead of two to deal with.
Here are a few of the better posts for the DML-based errors:
Basically, it looks like the problem boils down to fields with
unique constraints. Daniel's blog post does a great job of breaking down the debug process.
If you're getting this error while trying to deploy metadata, things are a bit tougher.
Debugging deployment errors is almost always painful in some way, but this particular error message with its two vague "<unknown>" references is especially cruel. The following steps can help to quickly get you on the right track to finding a solution.
Step One: Identify the Problem
This step is easy to start, but sometimes difficult to finish. Take a look at the error message, and I'll explain what I mean.
Error: objects/Opportunity.object(2098,13):duplicate value found: <unknown> duplicates value on record with id: <unknown>
Clearly the Opportunity object metadata I'm trying to deploy is causing a problem. Don't start Googling yet, though. There's more for you to learn about what, exactly, is failing.
Forget about the error "message" for a moment, and focus on the error location.
The important bit here is
(2098,13). This is the line and column number inside the
Opportunity.object metadata file where the trouble is coming from.
Here's what my file looks like near that line...
This tells me that the error seems to be specifically related to the
StageName picklist field that I'm trying to deploy.
Important: If this is EXACTLY the problem you're having, skip to the end to see a potential solution.
Step Two: Isolate the Problem
Deployments are tricky, complicated things with many dependencies to contend with. When debugging metadata errors that are not related to Apex code, simplify things by removing everything from your deployment package except the component that's giving you trouble (and its direct dependencies, of course).
If you're still getting the error after doing this, then you've successfully started the process of isolating the problem. In my case, I can try to Isolate things further by removing the entire
StageName field and re-trying my deployment.
After removing that field, my deployment finally succeeds! This conclusively proves that something I'm pushing in the
StageName field is causing the error.
Remember, this example is ONLY relevant to my situation. The metadata components that are giving you trouble will most likely have different names, be different types, etc. This is just an example.
Step Three: Start Digging
Now that I've conclusively isolated the problem to a particular field on a specific object, I can start digging deeper. Here's what I've found to be the most common possible causes.
For Picklist Fields...
If you've narrowed things down to a problem with a Picklist field, make sure that your target org does not already have an inactive picklist field with the same name as one of the ones you are trying to deploy.
If this is the case, you'll need to delete the picklist value. Once you do this, your deployment should proceed (until the next error sneaks up on you, of course).
For Other Problems...
If your problem is not related to a picklist field, the general consensus seems to be that there are some strange situations that come up if Field History Tracking is enabled on the object that is giving you trouble.
Here are the best solutions I've found related to "Field History Tracking" being a possible issue.
Debugging Salesforce errors is never fun. It's especially not fun when the error messages are vague, like the unhelpful "duplicate value found: <unknown>..." error message. This post has provided a framework for debugging this type of error, whether it came from a DML or a metadata deployment operation.
The steps from the "Deployment Error" section are useful for debugging other types of non-code deployment related errors. Hopefully you've found them useful. As time goes by, if additional "flavors" of this error show up, I'll update this answer to include them. Good luck out there!