Assuming that the "same ID" is 100% consistent, there's a simple, non-scripted way to filter out all the duplicates in one go, but I don't know if you can do it in FileMaker 6 like you can in a current version. What you do is take an empty copy of this file (a clone created in save a copy as...), set a Unique values validation rule on this ID with "validate always enabled", then import all the client records into this clone. The unique values validation then filters out the records with duplicate ID's during import and you'd then swap this file for the original after using the import to omit all the duplicates.
This is so much simpler than a looping script, that you might consider downloading a free trial of FileMaker 11, convert a copy of this file to FileMaker 11 by opening it, do the import method to filter out the duplicates, then export the clean data as a tab deliimitted file so that you can then use a clone of your FileMaker 6 file to re-import the cleaned up data.
If you try that and get it to work, you might show your bosses the trial copy and point out a few of the improvments that are now possible...