I reported the issue prematurely – today I found the donkey in question in the Nether. Previously, I had witnessed it walking opposite the Nether portal, so I discarded that explanation as a possibility.
Please close this report.
I can confirm this in 13w21a SMP. As Woody noted, the original horse will not obey the player's movement commands.
Additionally, the duplicated horse (not the original horse) despawned shortly after dismounting.
I reported the issue prematurely – today I found the donkey in question in the Nether. Previously, I had witnessed it walking opposite the Nether portal, so I discarded that explanation as a possibility.
Please close this report.