Buggy expounded:

Looks like maybe seeds were mixed. While it is evident that the large
plant is a cuke, there are also leaves of a watermelon plant in the
lower left of the attached picture. I highlighted it on your picture and
re-attached it.

Good catch! Yep, those are watermelon leaves.
--
Ann, gardening in Zone 6a
South of Boston, Massachusetts
e-mail address is not checked
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