Command in the [TclX] package to return the logical union of the two specified lists.  Any duplicate elements are removed.

Usage:
    :   '''union''' ''lista listb''

Example:
  package require Tclx
  union {a b c d e} {d e d f g c} ;# ==> a b c d e f g

Note that duplicate elements are removed and the resulting list is sorted.

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This functionality exists in [tcllib] in the [struct::set] package.

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I believe this can also be implemented in Tcl without an extension via:
======
proc union {a b} {
    lsort -unique [concat $a $b]
}
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