DataflowVariable can only be assigned once

Issue

You get the following error, connected to a process

Error executing process > 'PROCESS'

Caused by:
  A DataflowVariable can only be assigned once. Only re-assignments to an equal value are allowed.

Possible source

You forgot the parentheses around a function like flatten():

PROCESS
    .out
    .vcfs
    .flatten

Recommendation

Use static typing with Nextflow 26.10 or later. nextflow lint then reports the missing parentheses at the line where they are missing (line numbers refer to the full script):

Error main.nf:14:5: Unrecognized property `flatten` for type Channel<Set<Path>>