Dispatch App : Fix setting of dispatch plug from command line

The same dispatcher can be added to the `dispatchers` list multiple
times, once from the `dispatcher` parameter, and again from the
`alternateDispatchers` parameter.

That in turn resulted in the wrong dispatcher object being the one
updated by the settings from the command line.

The fix here is to prevent the duplicate addition of the same
dispatcher.

Changes.md

@@ -11,6 +11,7 @@ Fixes
 - Viewer :
   - Fixed visualisation of Cycles point light size.
   - Fixed visualisation of Arnold light gobo textures with scaled UV coordinates.
+- Dispatch App : Fixed bug that prevented setting specific dispatcher plug values from the command line (#5434).
 
 1.2.10.1 (relative to 1.2.10.0)
 ========

apps/dispatch/dispatch-1.py

@@ -186,7 +186,10 @@ def _run( self, args ) :
 			return 1
 
 		if args["gui"].value and len(args["alternateDispatchers"]) :
-			dispatchers.extend( GafferDispatch.Dispatcher.createMatching( " ".join( args["alternateDispatchers"] ) ) )
+			pattern = " ".join( args[ "alternateDispatchers" ] )
+			for name in GafferDispatch.Dispatcher.registeredDispatchers() :
+				if name != dispatcherType and IECore.StringAlgo.matchMultiple( name, pattern ) :
+					dispatchers.append( GafferDispatch.Dispatcher.create( name ) )
 
 		dispatcherNames = {}
 		for dispatcher in dispatchers :