Fix issues with file header fields getting repeated (cloned)

e.g. when using the JS FileBase.update() method
14 jobs for master in 8 minutes and 25 seconds (queued for 13 seconds)
Status Name Job ID Coverage
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passed jsdoor-freebsd #22172
FreeBSD

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passed jsdoor-linux #22173
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passed jsdoor-windows #22175
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passed sbbs-freebsd #22167
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passed sbbs-linux #22162
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passed sbbs-windows #22165
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passed sexpots-freebsd #22174
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passed sexpots-linux #22163
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passed sexpots-windows #22166
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passed syncdraw-freebsd #22170
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passed syncdraw-linux #22171
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passed syncterm-freebsd #22168
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passed syncterm-linux #22164
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passed syncterm-windows #22169
FreeBSD

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