What must be done to obtain a unique schema property?

In a sync project for data exchange with an application based on an MSSQL database, we use the sync project for MSSQL databases.
The mapping is set up so that the desired data can be determined and transferred. However, when we run the first simulation, the following message appears

The schema property <Property> does not have unique data. 
The value (<Property Value>) occurs more than once and all will be ignored. 
This will result in empty references

What is the reason for this? What can we do to resolve this warning?

Thank you for any advice.