There are circumstances where you only want one Dispatch open at a time for a given Machine. For example, if you already have a Machine down for a Code Red Dispatch, even if there is another concurrent issue, you don't want double downtime accruing. For those Dispatch Types, check the 'Allow only One Dispatch per Machine Open Simultaneously' box in the Dispatch Detail page (the Dispatch setup for each individual Dispatch Type). This will prevent multiple Dispatches being created against a Machine at the same time.
There are other circumstances where it makes sense to allow more than one open Dispatch against a Machine. For example, an Operator may be able to perform portions of multiple overlapping TPMs on the same machine at the same time. Perhaps there is also an open Kaizen Dispatch on the same Machine. For some Dispatch Types, you will not want the 'Allow only One Dispatch per Machine Open Simultaneously' box checked in the setup for those Dispatch Types.
This functionality can be enabled or disabled by individual Dispatch Type from the SETUP menu. Ask your Team L2L Admin or your Site L2L Champion if you're not sure which setting is appropriate.