Is it ever allowed to list your destination airport as the IFR alternate too?
You would never list your destination as an alternate (that wouldn't logically make sense), but there are situations where having an alternate is not required:

At least one destination alternate aerodrome must be designated and specified in the both operational and ATS flight plans if a flight under IFR is going to be made unless one of the following applies:

after taking all relevant factors into account, it is reasonably certain that VMC will prevail for the approach and landing and there is more than one independently useable runway available which can be expected to be useable at the ETA and at least one of those runways has an instrument approach procedure

the aerodrome is isolated, in which case a destination alternate is not required and instead additional fuel must be carried. a Point of No Return (PNR) must be determined and this must not be passed unless an assessment of the prevailing weather, traffic and other operational conditions indicates that a safe landing can be achieved at the ETA.



Source: https://www.skybrary.aero/index.php/Alternate_Aerodrome (ICAO Annex 6 Chapter 4)