You can not use payment links more than once. 

The links are for one-time use. You can generate as many payment links as you need. 
Each generated link can only be used to take a single payment.

A consumer will not be able to make a double payment, they will be greeted with a message notifying them this link has already been paid.

We strongly recommend using unique payment references for each transaction.

• The payment reference is your reference for the PayByLink transaction.
• This value should be unique in order to protect your consumers against duplicate transactions.
  Note: If your integration requires reusing a payment reference, please contact our Dev Support team so we can check whether this setting is supported for your account.

Note: Expired or cancelled links cannot be used. 
For more information, see When does a Payment Link expire?

The Consumer Reference is a required field to be entered when creating a payment link. 
This is a unique reference to anonymously identify the consumer.
Note: The consumer reference is case sensitive.