You can do kind of reset mechanism to clear last choice after a little delay.
See I placed first option as dummy and if you send same data, it will be chosen but you then filter that out as no action should follow.
[{"id":"b6da561d9b14e38a","type":"ui_dropdown","z":"09deee63c1b960f7","name":"","label":"","tooltip":"","place":"Select action","group":"08ac9b242ae51e8c","order":11,"width":0,"height":0,"passthru":false,"multiple":false,"options":[{"label":"Select action","value":-1,"type":"num"},{"label":"Action one","value":1,"type":"num"},{"label":"Action two","value":2,"type":"num"},{"label":"Action three","value":3,"type":"num"}],"payload":"","topic":"topic","topicType":"msg","className":"","x":380,"y":100,"wires":[["7c7631f7ac46e4b3"]]},{"id":"737447316fbbd0b8","type":"debug","z":"09deee63c1b960f7","name":"action","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":770,"y":100,"wires":[]},{"id":"7c7631f7ac46e4b3","type":"switch","z":"09deee63c1b960f7","name":"","property":"payload","propertyType":"msg","rules":[{"t":"neq","v":"-1","vt":"num"}],"checkall":"true","repair":false,"outputs":1,"x":570,"y":100,"wires":[["628c83b629bf0d2f","737447316fbbd0b8"]]},{"id":"4e3cf42d291060b2","type":"link in","z":"09deee63c1b960f7","name":"reset dropdown","links":["2e0921a373d3faab"],"x":285,"y":160,"wires":[["b6da561d9b14e38a"]]},{"id":"2e0921a373d3faab","type":"link out","z":"09deee63c1b960f7","name":"dropdown reset","mode":"link","links":["4e3cf42d291060b2"],"x":815,"y":160,"wires":[]},{"id":"628c83b629bf0d2f","type":"trigger","z":"09deee63c1b960f7","name":"wait and reset","op1":"","op2":"-1","op1type":"nul","op2type":"num","duration":"1","extend":false,"overrideDelay":false,"units":"s","reset":"","bytopic":"all","topic":"topic","outputs":1,"x":660,"y":160,"wires":[["2e0921a373d3faab"]]},{"id":"08ac9b242ae51e8c","type":"ui_group","name":"Default","tab":"2acb95821445c8af","order":1,"disp":true,"width":"6","collapse":false,"className":""},{"id":"2acb95821445c8af","type":"ui_tab","name":"Home","icon":"dashboard","disabled":false,"hidden":false}]
And if the delay is not good choice for reset, you can make any logic to figure out when to send reset to dropdown.