That means more than one column has been mapped to the same measure. In import sources where dates are in every row, each measure needs its own dedicated column.
Adjust your mapping selections so each measure is mapped to a single column in the mapping window. You may need to cancel your import and adjust the import source so each set of measure values is in its own column and not spread across multiple columns.
In the following example, two columns were mapped to User Budget Unit 2. Changing one of the column mappings to a different measure will correct the issue.
Closing the message shows one of the impacted columns highlighted in red. The column is changed to User Budget Amt 2, as shown in the last image.
After the change, the import mapping is valid and the import can be processed.
