1. Check whether you are affected by this known issue – https://success.salesforce.com/issues_view?id=a1p30000000T5hTAAS. If yes, contact Salesforce support.

2. DML Error:
If you’re getting this error from a DML operation, then check the below

1. Check whether there are any fields with Unique attribute selected. If yes, check whether you are entering unique value to this field while doing data loading. Also, check whether any processes are updating this unique field value. Setting debug log, helps to find it or search through the metadata.

3. Deployment Error:
If you’re getting this error while trying to deploy. then check the below

1. Check whether there are any fields with Unique attribute selected. If yes, check whether test class is creating unique value for these fields.
2. Check whether you are trying to deploy picklist field. If yes, make sure the value is not inactive in the target org.