how to merge and will merging create duplicates?

You can NOT merge gedcoms on tribalpages - you can overwrite and existing one. If you want to merge, you best solution is to you family tree type software that is resident on your computer. TWo of the best for this purpose are Legacy and Roots Magic (I think RM does the best merge)

Once you have merged the two gedcoms, you create a new gedcom of the merged file and overwrite the tribalpages site. If you have not changed peoples names - all pictures should reattach. BUt those where you changed a middle initial to a full name or such may not, and you will have to reattach them.

It really is very easy. I do it all the time. I would not suggest using FTM as it produces a non-standard gedcom which can cause problems