Although not the most practical, I found this to be simplest and cheapest solution, provided you have space to spare, and some time. After trying to figure out what course to take in merging libraries (long story, but I have three of them), I just decided to create a temporary folder in my external and drag the contents of the other two iPhoto libraries in there. Took me about 20 minutes, but since this is a one time thing, I didn't mind.
As for duplicates, iPhoto would prompt if the photo already existed, and I just simply instructed to not import.
The steps:
1. create a temporary folder to host all the images being migrated
2. open the source iPhoto library by double clicking the library folder
3. drag the photos you wish to migrate into that temp folder (you can drag entire and multiple events from events view)
4. quit iPhoto and double click the target iPhoto library (you can also option+launch the dock icon to manually search for the library).
5. drag the photos from the temporary folder into the iPhoto icon on the dock.
Gets the job done.