I have an agent queue in VSTS with several agents and I have a build assigned to this queue. Is there any way of telling VSTS that only one instance of the build should be r
You can't configure this at the Build level (You can vote for this feature here), but perhaps you can accomplish what you want at the phase level.
Look at the Parallelism options under each Phase. Looks like you'll want to set this to "None" if you are seeing behavior contrary. "None" is the default option for new builds.