evaluator: cannot add field $X: was already used

[myitcv]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.0.0-20221201203158-959dc46d5ac9

       -compiler gc
     CGO_ENABLED 1
          GOARCH arm64
            GOOS linux
             vcs git
    vcs.revision 959dc46d5ac976bd2fc9b6f4bf0cd596c00fd0c5
        vcs.time 2022-12-01T20:31:58Z
    vcs.modified false

Does this issue reproduce with the latest release?

Yes

What did you do?

See https://github.com/cue-unity-private/2fa7c0c443a4b1b3274b32b2a060e551e22e94b2ca29ec970a7d367c888baf23/issues/1

Like #2163 and #2113, this is a regression vs v0.4.3

cc @bttk