JabRef can help you merging entries of your database.
First, select the two entries to be merged. Then select the menu Quality → Merge entries…. The Merge entries window will pop-up.
The fields of the two entries are displayed side-by-side on the upper part of the window.
The differences between the two entries can be emphasized through the drop-down menu located at the upper right-hand corner of the window. Five ways of displaying the differences are offered:
In the central column, a radio button allows you to select which side to keep for each field: the left side, the right side, or none. By default, the left entry is kept, and any fields not present in the left entry are obtained from the right entry.
Based upon your selection, the merged entry is shown, both as a preview (on the left) and as source code (on the right).
If you right-click on the preview, you can Print entry preview or Copy preview.
Finally, after selecting which fields to keep, you can decide to Merge entries. Alternatively, you can press Cancel.
See also: Find duplicates