Do I pay double VAT through OSS if I have already paid VAT in EU countries?

No: The OSS applies only to cross-border sales and local VAT is handled separately.

No, there is no double taxation. The OSS filing only applies to distance sales (sales to customers in other EU countries). Local sales are handled through the standard VAT return of the respective country. This means you only pay VAT once per type of sale: either locally or through the OSS system.