You should restructure your database so that you do not have multiple email addresses. Having those duplicate records will create many issue for you in addition to this one.
You can set up a related table of groups plus a third, join, table and then you can link one record with a given email address to multiple groups and yet have a given group list many email addresses.
Let me explain why the duplicates are there. My database contains students that belong to a given research lab that has only one supervisor. The duplicates come from the fact that many of these students have several research grants from which they draw funds, hence more than one entry in the db. It is necessary to keep these funds separate so I have to have duplicate entries.
You still do not have to have duplicate email addresses in your data. Are these email addresses for the student, the supervisor or the grant administrators?
What you have described so far reads like a series of one to many relationships:
But if a given grant may be linked to more than one student, you can have:
---< stands for one to many