According to the Award Wallet Blog, after AA switched to dynamic pricing, booking a roundtrip award appears to require slightly fewer miles than two one-way awards, suggesting there may be a discount. Using LAX-LHR as an example, a one-way costs 67,000 miles, while a roundtrip costs 112,000 miles, which works out to 56,000 miles each way.

The discount also varies by cabin.

If you have a relevant booking in mind, try searching it as a roundtrip.