JabRef can help you merging entries of your library.
First, select the two entries to be merged. Then select the menu Quality → Merge entries. Alternatively, select the right-click menu 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 left-hand corner of the window. Five ways of displaying the differences are offered:
plain text: as-is, no emphasis
show diff - word: differences are shown in the right entry. Full words are struck out in red if they are removed from the left entry or underlined in blue if they are added to the right entry.
show diff - character: differences are shown in the right entry. Individual characters are struck out in red or underlined in blue as above.
show symmetric diff - word: differences are shown on both sides. Words are underlined and displayed in color.
show symmetric diff - character: differences are shown on both sides. Characters are underlined and displayed in color.
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.
Finally, after selecting which fields to keep, you can decide to Merge entries. Alternatively, you can press Cancel.