For the basics on importing glossaries into Multiterm, read this blog entry: Importing Excel Glossaries into Multiterm.
When you're ready to import your glossaries, you will see two standard import definitions in Multiterm, called "Default import definition" and "Synchronize on Entry Number". Neither one of these can be used to avoid duplicate entries, but they can be used as the basis to create a new one.
1. Right click on Synchronize on Entry Number and choose Duplicate. This will make a copy of the import definition.
2. Right click on Copy of Synchronize on Entry Number and choose Edit. This will open the Import Wizard, which lists the steps the wizard will go through. The change we need to make to avoid duplicate entries is on Step 4 (Exclude invalid entries from the import), so click Next.
3. Give a more user-friendly name to your new import definition. I have chosen "Avoid Duplicates" for this example. You can also add a description. Click Next.
4. Choose the file that contains the entries you want to import. Alternatively, you can leave these fields blank and choose your import files later. Click Next. When the wizard asks for an Exclusion File name under Validation Settings, type any name.
5. This takes us to the key step, Step 4. The default here is "Synchronize entries on entry number", but choosing this will result on duplicate entries if for some reason your glossaries got rearranged. Select "Synchronize entries on index term". Click Next.
6. Now we choose our Advanced Options for synchronizing on index terms. To keep things simple, I leave the Index as Source, and under "Index term does not exist in the target termbase", leave "Add import entry as new", because otherwise the new entries will not be added to the termbase. For "Index term exists in the target termbase", change the Action to "Omit import entry" if you want to skip it altogether, or choose one of the other options based on your needs.
Click Next a couple more times and the wizard closes, leaving you with a new import definition like this:
Finally, to make sure you can use this new import definition with any of your termbases, be sure to save it by right-clicking and choosing Save. This will save a file with an xdi extension, which you can later load into any other termbase, by right clicking the Import Definition area and choosing Load.
Now, whenever you need to reimport a glossary and you want to avoid getting duplicate entries in your termbase, run this new import definition by right-clicking it and choosing Process.