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“Ton” is the standard spelling for an Imperial (i.e. old British) ton, a US ton, or one of the many other sorts of ton (such as a long ton, a short ton, a net ton or a displacement ton).
The exception is the “tonne”, which is the same thing as a metric ton. To avoid confusion, never use the spelling “tonne”, but always say “metric ton” or whatever other sort of ton you mean.
The form “metric tonne” is wrong.