  Given an extendible spacetime one may ask how much, if any, uniqueness can in
general be expected of the extension. Locally, this question was considered and
comprehensively answered in a recent paper of Sbierski, where he obtains local
uniqueness results for anchored spacetime extensions of similar character to
earlier work for conformal boundaries by Chru\'sciel. Globally, it is known
that non-uniqueness can arise from timelike geodesics behaving pathologically
in the sense that there exist points along two distinct timelike geodesics
which become arbitrarily close to each other interspersed with points which do
not approach each other. We show that this is in some sense the only
obstruction to uniqueness of maximal future boundaries: Working with extensions
that are manifolds with boundary we prove that, under suitable assumptions on
the regularity of the considered extensions and excluding the existence of such
''intertwined timelike geodesics'', extendible spacetimes admit a unique
maximal future boundary extension. This is analogous to results of Chru\'sciel
for the conformal boundary.
