The Doctor, at the beginning of "Asylum of the Daleks", said that the planet was "Skaro. The original planet of the Daleks." That leaves no room for doubt about which planet it was. On TV, no explanation has been given of how that fits with the events of "Remembrance of the Daleks".
According to "War of the Daleks" (BBC Eighth Doctor Adventures novel), the Dalek Supreme found out what would happen to Skaro, so it had a nearby planet terraformed to look like it to fool the Doctor into destroying that planet, thus leaving the real Skaro alone.
- Even if that novel is not taken into account, the Doctor is a time traveller. No further explanation is necessary, really. (Actually, the prequel seen only in the US on iTunes reveals the Doctor himself didn't travel to Skaro, but was transported there. Still, he could have been transported through time to a point before the planet was supposedly detroyed.)
- When Skaro was supposedly detroyed in "Remembrance of the Daleks", the dialogue implies that its destruction also involved time travel, so we don't know when that destruction was meant to have occurred.