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If they are plain text files, merging them will be difficult. If they had been downloaded in a format that is compatible with EndNote (web version) you could upload them to EndNote and then remove duplicates as well as generating a bibliography.

In Microsoft Word, you can sort your list alphabetically and then manually remove duplicates. Microsoft gives these instructions:

Sort a list alphabetically in Word

This is from the FAQ section of Bibliometrix website:

Q6 - How can I upload multiple export files in Biblioshiny?

You have just to put your export files into a single zip-compressed file and then import it with Biblioshiny.

https://www.youtube.com/live/xQg7Eg4fW54?si=_TL27XkTON4Vq4-J