==> language/english/sentences/repeated.words.s <==
Five "had"s in a row:

The parents were unable to conceive, so they hired someone else to
be a surrogate.

The parents had had a surrogate have their child.

The parents had had had their child.

The child had had no breakfast.

The child whose parents had had had had had no breakfast.

It is arguably possible to construct sentences with arbitrarily many
repetitions of a word.  For example:

1. Bulldogs fight
2. Bulldogs bulldogs fight fight.
    (i.e., bulldogs (that) bulldogs fight, (themselves) fight)
3. Bulldogs bulldogs bulldogs fight fight fight.
    (i.e., bulldogs (that) bulldogs (that) bulldogs fight, (themselves) fight,
    (themselves) fight)
...
n. etc.