[rubenhak]

SQS-FIFO should let you process task once & only once. Just make sure you configure timing parameters correctly.

DynamoDB has triggers that get fired upon changes. That would strongly help with eventual consistency implementation (which i strongly recommend). But with this you should write Lambdas. Check how well is Lambda Java supported.

Are you sure you want to use EC2 directly? Why not to use ECS? This would let you focus more on the business and less on infrastructure