Limited relationship issue

Salac If you don't have unique values in any tables so try creating a Bridge Table as below:

Bridge Table = DISTINCT(TableName [Key NB])

Then join this Bridge table to both of the remaining tables.

Related Function always work in the table that is at many side of the relationship . so there should be one to many relationship b/w the tables.
Try creating a Bridge table for storing unique records and then connect that table to both the tables 
using one to many relationship.

I think that the problem is you have values in both tables which don't appear in the other table, resulting in a blank row being added to both and a many-to-many relationship.

The easiest solution would be to use LOOKUPVALUE rather than RELATED, using your key column to do the lookup.