When importing using Design Transfer, if what you are importing already exists in the target database you will, depending on what you are importing, be prompted to choose whether or not you would like to import using a modified title or keep what already exists in the target database and not import the Attribute/Object/Process.
You have chosen "Do not create new Class Type and keep only existing one" when importing but you are still getting the below error message pointing out that it's trying to import something that already exists in the target database.
"A Internal Exception has occurred while committing AttribyteType xxxxxx
Attribute_xxxxxx already exists on Class xxxxx on xxxxxx in Module Config"
This is most likely due to having two attributes with the same Name but different Guids in the two databases that you are exporting/importing in between.
The import into the target database will initially do a Guid scan to see what already exists in the database, this is how it determines whether or not to prompt you with the above question. However upon the import when it is adding on the Attributes it will try to add a duplicate Name of an Attribute and therefore error.
This has been logged as problem 5194. If you are experiencing this problem, please log this with your Support Provider referencing problem 5194.
The suggested solution to this is to delete the relevant Attribute in the target database and then redo the Import.
Only if that isn't possible can you change the Guids in the .XML file to match the Guiid in your target database. What really needs to be considered with this solution though is that the Attributes will need to be IDENTICAL. They will need to have the same Data Type, same Character length, same behaviour and so on. This is very important to check as otherwise you will corrupt the Attribute.
Please also always take a fully restorable backup of your target database before attempting to import using Design Transfer.