Check the design of your database. You’re probably trying to create a many-to-many rela-
tionship without using a junction table. If neither field is a primary key or has a unique
index, Access knows that you can enter duplicate values on both sides of the relationship; as
a result, it’s impossible to define the relationship properly. If necessary, define a unique index
for one of the fields.
Secrets of the Office Masters: Creating Links to
External Databases
In some cases, you might want to incorporate the contents of an external database file (or a
collection of related files) into an Access database. ...
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