Payment didn’t upgrade member but created new account

I’m using WP membership plugin for the registration and payment of a professional society. The members register as simple members and can then either pay by bank transfer (remaining simple members) or by paypal online payment (change of membership level). However, when a member pays through his paypal account having a different email address than the one he registered with, the system creates a new member instead of upgrading the actual account. If both emails are identical, the problem doesn’t arise. How can I merge the two accounts without losing trace of the transaction?

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