I'm not sure I properly understand ネタ被り, but I think the joke (aside from the obvious joke of who is grabbing who) is that the punchline wasn't supposed to be until you see Haruhi holding Mikuru in the last panel, but Nagato gave it away early.

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Given Koizumi's reaction, I think it was Haruhi's attempt to get Koizumi and Mikuru to hold each other, but Nagato essentially beat them to the punch.

BakaHoushi said:
Given Koizumi's reaction, I think it was Haruhi's attempt to get Koizumi and Mikuru to hold each other, but Nagato essentially beat them to the punch.

But Haruhi has already claimed Mikuru for herself.