local.tee

local.tee writes to a local variable and leaves the value on the stack for further use.

(module
  (func (param $x i32) (result i32)
    (local $y i32)
    (i32.add
      (local.tee $y (local.get $x))
      (i32.const 1)))
)

There is no global.tee. To set a global and keep the value, use local.tee into a temporary then global.set:

(module
  (global $g (mut i32) (i32.const 0))
  (func (param $x i32) (result i32)
    (local $tmp i32)
    (global.set $g (local.tee $tmp (local.get $x)))
    (i32.mul (local.get $tmp) (i32.const 2)))
)

Instruction Reference

• Local & Global Instructions — local.get, local.set, local.tee, global.get, global.set