A client paid the same invoice twice and then deducted the money from the following months payment. how do I show this in QB’s?

I'll help you record the payment in QuickBooks and show it according to your needs, dave193.
 

If this is an overpayment, we can record the excess payment as a credit. Here's how to record the total payment: 
 

1. Go to the + New button.
2. Select Receive payment.
3. Enter your client's name, select the invoice and enter the total amount in the Amount Received field.
4. Hit Save and close.

Then, apply this for their future payments. Here's how:

1. Create the invoice.
2. Once done, select Receive payment.
3. Tick the new invoice and credit.
4. Review the amount and click  Save and close.
   
    

 Also, we can configure customer credits so that they automatically apply to future payments. We can simply turn it on by going to the Gear icon> Account and Settings> Advanced> Automatically apply credits. 

For additional tips and guides about handling customer credits and overpayment, feel free to utilise this article: Learn what to do in QuickBooks Online when a customer overpays you.

Lastly, we can visit this link to learn about managing your customer's data, income, and sales forms: Topics about your company's income and customers.

Please let me know if you have more concerns about recording customer transactions. I'm happy to help you anytime. Take care and stay safe.