"field not allowed" regression in evalv3 when duplicating definitions

[mvdan]

# With the old evaluator.
env CUE_EXPERIMENT=evalv3=0
exec cue vet -c

# With the new evaluator.
env CUE_EXPERIMENT=evalv3=1
exec cue vet -c

-- input.cue --
package p

#Context1: ctx: {}
#Context2: ctx: {}

#Config1: cfg: #Context1
#Config2: cfg: #Context2

#Schema: sch: #Config1

#Config: #Config1 & #Config2

let config = #Config
out: #Schema & {
	sch: config
}

As of 46fc54a:

# With the old evaluator. (0.010s)
> env CUE_EXPERIMENT=evalv3=0
> exec cue vet -c
# With the new evaluator. (0.029s)
> env CUE_EXPERIMENT=evalv3=1
> exec cue vet -c
[stderr]
out.sch.cfg.ctx: field not allowed:
    ./input.cue:7:16
    ./input.cue:3:12
[exit status 1]

Note that deduplicating either the Context or Config schemas, which should be a no-op, fixes this issue. So it seems like something is broken with the new evaluator.

Perhaps this is the same underlying issue as #3639, although we haven't reduced that one yet.