Could he be the John Lackey who enlisted in company A? The index to the compiled military service records does have different cards for them, with no cross-reference. But the National Archives did not find a compiled service record for the John Lackey who served in company G.
He was a private in company G. [source: 1]
(I have no other information about him.)
1 index to compiled service records of volunteer Union soldiers who served in organizations from the state of Pennsylvania (John Lackey)
[ordered compiled service record from NARA, 5 August 2015, order G11-234505210E]