How can I restore a purchase that I did twice by mistake?

If you have received an invoice from Apple, on every invoice from the Apple Store and/or iTunes Store always states what you have purchased (and/or concerns a subscription) and the amount.

Tap on Report a problem on the invoice.

Or try to ->  Request a refund for apps or content that you bought from Apple.

Request a refund for apps or content you've bought from Apple – Apple Support (UK)

If you have requested a refund, you must wait until Apple has approved. Then you will receive a message from Apple.

Refunds are not guaranteed. For more information, see the Terms & Conditions.

Try to cancel the subscription -> If you don't want to continue a subscription you need always to cancel that subscription in time, see the following Apple articles:

If you want to cancel a subscription from Apple

If you want to cancel a subscription from Apple - Apple Support

If the subscription that you want to cancel is not listed

If the subscription that you want to cancel is not listed – Apple Support (UK)

What happens when you click on Manage Subscription?

Is it possible to cancel the subscription?

But my purchase has no invoice.

Most likely you will still receive the invoice.

Check tomorrow or so.

Based on -> FaceApp -> FaceApp PRO

Payment will be charged to iTunes Account at confirmation of purchase. Subscriptions can be managed and auto-renewal can be turned off in Account Settings in iTunes after the purchase. Any unused portion of a free trial period, if offered, will be forfeited when the user purchases a subscription to that publication, where applicable.

Get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)