Scenario 1. You could have a period of more than 2 yrs reckonable service, left, rejoined (before 6 April 2005) and then been subsequently medically discharged. If the pension for the first period of service is not aggregated (joined) with the second, you could have a preserved pension for that and a SIP or SAP/GIP in payment. I would hope that someone on the welfare side of the house would have pointed out the benefits of aggregation - certainly the FAR brief would have done so.
Scenario 2. It is not totally out of the question to be medically discharged and to rejoin the Armed Forces at a later date - but it is very rare. The SIP part of the pension would be abated (the RBL will know what happens to the SAP) until you leave again. As the pension for the first period of service has been paid out, it cannot be aggregated with the new pension entitlement so, to get an immediate pension you will need to give 22 yrs (OR) ir 16 yrs (officer) service. If you left before that point (but with more than 2 yrs new reckonable service) your pension would be preserved.
If this doesn't answer your question - or inspires more - send me a PM setting out your circumstances.
I took it to mean the OP meant for the same injury and should have known better, particularly as I get a War Pension and eventually an AFCS award. Both for different periods/types of service and different injuries.