For the most part it was common to the point of universality. In a war as large as WW1 there will always be counter examples you can dig up, but by and large useful or expensive parts of kit were very often taken from the dead.
Things like gas masks, helmets and guns were very commonly stripped from casualties during battle, as were ammunition, food and water and bits of uniform or kit that needed to be replaced immediately.
Soldiers would also go through the pockets and any bags/pouches that a dead man had looking for souvenirs or anything of value. This was a little frowned upon in the British and Dominion sources I’ve seen as it was seen as distasteful/disrespectful, at least when done to ones comrades. Did soldiers do things like steal the boots from dead men? Yeah, it was relatively common if time permitted. Uniforms themselves were a much less common thing to be taken, but again, not unheard of. Webbing and pouches, if in good condition were a useful thing to be reused.
Every army had units called salvage companies dedicated to scouring the battlefield for usable material/equipment. Primarily they were after metal, especially shell casings, and building materials for use in road and trench building.
And shells themselves were something well worth salvaging if possible.
There is an account by George MacDonald Fraser (of Flashman fame) who described what happened when a comrade was killed: “Not a word was said about Tich Little, but a most remarkable thing happened (and I saw it repeated later in the campaign) which I have never heard of elsewhere, in fact or fiction, although I suspect it is as old as war.Tich’s military effects and equipment — not, of course, his private possessions, or any of his clothing — were placed on a groundsheet, and it was understood that anyone in the section could take what he wished. Grandarse took one of his mess-tins; Forster, his housewife [sewing and mending kit], making sure it contained only Army issue and nothing personal; Nixon, after long deliberation, took his rifle, an old Lee-Enfield shod in very pale wood (which surprised me, for it seemed it might make its bearer uncomfortably conspicuous); I took his pialla, which was of superior enamel, unlike the usual chipped mugs. Each article was substituted on the groundsheet with our own possessions — my old pialla, Forster’s housewife, and so on — and it was bundled up for delivery to the quartermaster. I think everyone from the original section took something.” Quartered Safe Out Here Dividing up a dead comrade’s possessions
