Running two consecutive deployments against Marathon quickly enough can lead to a 409 error response containing the message
> App is locked by one or more deployments. Override with the option '?force=true'. View details at '/v2/deployments/<DEPLOYMENT_ID>'.
As the force option is only meant to be used for failed deployments according to the official docs (see the Force a Deployment section), I would argue that a more reasonable way to deal with this is to queue up all pending deployments and apply them successively. This also offers the possibility to "skip" waiting deployment requests as newer ones are overtaking.