This happens only under specific circumstances.  The base list must have a required lookup column/field that is multi-value.  The view must not include this column.

A required, multi-value lookup column is created to demonstrate the problem:

An example of the list data in standard view, showing the “Region” column:

I created a new view without the “Region” column to show the duplicates bug.  Standard view appears normal:

Same view, but when switched to Datasheet view, duplicate records show:

Same view in Export to Spreadsheet shows duplicates:

If the column is not required, everything works fine.  If the column exists in the view, everything is fine.  It is only when the required column is omitted from the view that you see duplicates.

If the column is included from the view, everything is normal during Edit in Datasheet mode:

Duplicates are now gone in Export to Spreadsheet as well:

There seems to be no fix for this problem.  The work-around is, if you have a required multi-value lookup column, to always add the column to views, especially datasheet views.  If a user creates their own view without the column, they may run into this problem.  User education is the only solution in this case.

Although this seems like a rare circumstance, it does appear that multiple SharePoint users are encountering this problem.  This affects both WSS 3.0 and MOSS 2007.