See below from inputs.conf, the first stanza is going to pick it up since the remote server is empty and only one stanza per port number is currently supported. You could override the sourcetype on a per event basis using regex. See this link - http://docs.splunk.com/Documentation/Splunk/latest/Data/Advancedsourcetypeoverrides
You could also change the port number.
[udp:// : ]
* Similar to TCP, except that it listens on a UDP port.
* Only one stanza per port number is currently supported.
* Configure Splunk to listen on a specific port.
* If is specified, the specified port will only accept data from that server.
* If is empty - [udp:// ] - the port will accept data sent from any server.
* Will generate events with source set to udp:portnumber, for example: udp:514
* If sourcetype is unspecified, will generate events with set sourcetype to udp:portnumber
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