Go to the amcatalog, "all" tab. Localize the part if it isn't already by right clicking on the name then "localize". Now you can right click on that localized part and choose "copy definition". Give it the new part name. These parts can be externalized again but be aware that if they have drawing views associated with them, the views will be lost. To keep both parts external, open the part you want to copy, save it as the new part name, and insert that new part into your assembly through the catalog.
You can also simply select the part so that the outline is a dashed line like AutoCad. Do the CTRL+C and then CTRL+V to paste a new one. The new part will be the same and in the model tree it will have the same name except with a tilde "~" in front of it. You can rename it and use it as you please. I use this method a lot when doing piping layouts and I have multiple lengths of the same sized pipe. I simply copy it and paste it then change it according to my needs.
Although the copy/paste method seems to work it is not the recommended way of creating new parts. I believe you may run into problems down the road by using this method. From what I understand, part insertion needs to be done through the amcatalog in order for the program to properly audit the part creation.
I haven't had any issues with that method....yet. I'll definitely keep that in mind.
However, I should have included one caveat in my explaination....I have modeled the part I copied in that drawing. I did not import it in via amcatalog. I haven't tried copying something that way.
