"Fixing Repeated Data Merge in InDesign Resulting in Excessive Pages"

When using the Data Merge feature in InDesign, I am encountering an issue where the content from my second document (B), which I am trying to data merge into my main document (A), gets repeated multiple times. Specifically, the merged data appears like 'aaaaaaaaab1aaaaaaaaab2' and continues in this pattern for all records of document B. This results in an excessive number of pages—up to 1000 pages. How can I resolve this issue so that the data merges correctly, without unnecessary repetition, and I can achieve the intended layout?