A diphthong refers to the phenomenon where two vowels are pronounced consecutively within a single syllable.

This house is warm even in winter because it has double-pane windows.

He repelled the enemy using the heavy rattan bow.

He was criticized for having a double standard.

These shoes have a double sole and are very durable.

Doublet is an important concept in linguistics.

He drew the heavy vine bow.

He holds dual citizenship in America and Japan.

His actions always have a duality.

Haplology is a part of linguistics, referring to the phenomenon where a part is omitted when the same syllable repeats.