People subscribe to a free plan once sometimes twice!

Hi Si Team

Sometimes people subscribe to a free plan “BUG All Members” once sometimes twice!
Kindly go into the back end and check the subscriptions and sort for “BUG All Members.”
And you should see what I mean.

eg

Sandy (Twice) id: 1639 & 1638
Donald (Twice) id: 1525 & 1524
Uwe (Once) id: 1519
Michael (Twice) id: 1497 & 1496
Andreas (Once) id: 1477

Any idea what is going on here?

thanks

Paul