Based on your description, you are working with a ... spreadsheet!  While
you might need to use separate worksheets for Swimming and Gym in Excel,
Access is a relational database, and designing your tables this way will
prove counter-productive.  As your post points out, you've already found one
of the ways this design doesn't work well.

If you have three tables like:

   tblPerson
       PersonID
       FirstName
       LastName
       StreetAddress
       ...

   tblActivity
       ActivityID
       Activity (this is your "Swimming", ...)

   trelPersonActivity
       PersonID
       ActivityID

you could use this to record each person a single time, then show which
Activity/Activities that person is connected to.

Why would you want to do this extra work?  Well, you've already found one
reason why, here's another ... right now you have only two activities.

Are you ready to re-design your tables, your queries, your forms, your
reports, your macros, and your code every time a new activity gets added?!
With the three table design I laid out, you could, instead, simply add one
new row to the tblActivity and use it, as needed, in that third table.

(and if you insist on using Access like a spreadsheet, look into UNION
queries ... this would give you a way to combine results from a query
against your Swim table and a second query against your Gym table...)

Good luck!

Jeff Boyce
Microsoft Office/Access MVP