This is not a case of papers being "mis-filed" - it is an ORIGINAL ERROR. Perhaps a Clerk making alterations? Difficult to understand how it happened - the men were NOT in the same regiment.
But if one of these 2 men is yours ... you will need a fine toothcomb to sort the service details!

JOHN BROOKE MARSHALL - born 1885 (Q3 Holborn) married EMILY MARGUERITE HARBOROW 28 July 1906 Islington. Home address: ISLINGTON.

JAMES MARSHALL - born c1881 married MARY HALL 29 October 1910 Bromley Kent.
2 children: ELSIE 27 Dec 1910 & PERCY 5 July 1912. Home address: BROMLEY.

The papers are filed (in the WW1 Service Records - WO 363) under John Brooke MARSHALL, who enlisted 10 May 1916 into the Suffolks (33725).
JAMES MARSHALL joined the Essex Regiment (200808) & was transferred to the KOYLI (235289).

The KOYLI number is where the problems start - it has been added to John Brooke's papers, and the Bromley details inserted.
James has had "Brooke" added to his name. But on discharge in 1919 he is just JAMES.

Anc****y, MAR, pages 46304-46325

If one of them is yours, I wish you the Best of Luck!