Steps to Reproduce the issue:
Here an object may be listed more than once like Opportunities, contacts etc.
- Go to Setup || App Setup/Build || Create || Report Types
- Click on 'New Custom Report Type' button
- Select the Primary Object, e.g. Accounts
- Enter the required information on the Step 1 of the Report Type creation wizard and click Next
- On step 2, click on 'Click to relate another object' to select a related object
It happens when an object has various lookup relationship fields and the 'Related List Label' for all those relationship fields are similar.
For example: On Opportunity object, the user may have created multiple lookup fields(lookup to Account). By default, the 'Related List Label' becomes 'Opportunities' for all the lookup fields created.
When you create a report type on the Parent object (Account), it shows Opportunities listed more than once due to the multiple look up fields.
The resolution is to change the ‘Related List Label’ for these fields to something different, which will then not display this name listed more than once. This in turn will avoid the confusion and we will be able to create a Report Type with the correct child object relationships.