I'm pretty sure Alamy would say they do not regard themselves as a microstock site and would not encourage contributors to upload identical images to their site and microstock sites.
Having said that, they do nothing to prevent it and it is down to the individual contributor what they submit. Indeed, since they now have a policy of encouraging pretty well anybody with a half-decent camera (or iphone) to submit, it could be argued they are opening up the contributor market even more than some microstock sites. Their QC policy of only inspecting a small sample of of submitted images tends to reinforce this, given that it was originally designed to cater for submissions from experienced professional photographers, rather than people who are still low on the learning curve.
Alamy's 15 minimum nominal price per image would tend to suggest that identical images which are also available on microstock would never sell on Alamy. This is not totally the case, but it would be surprising if many clued-up purchasers bought from Alamy when they could easily and cheaply go elsewhere for the same image.
Alamy's strength is in RM soft editorial, UK especially but worldwide shots too. I try to play to the strengths of the different agencies and submit to Alamy only editorial - those images which it does well with. They may not sell in quantity but just occasionally they provide a very good sale, which makes microstock prices look silly. I don't think it is my interest, or indeed in the interest of new contributors, that Alamy goes down the line of becoming just another microstock, so my stance is to resist the temptation to put on Alamy all the stuff you have submitted elsewhere.