Duncan sits next to 4 phonetic neighbors — Donovan, Dunstan, and Declan are the closest matches by sound. Most share Duncan's Scottish cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"dark warrior"
"land of purple dye"
"Yahweh saves"
"from the river Blaan"
"craftsman of fermented beverages"
"Frenchman, man from France"
"stellar explosion, burst of light"
"one who works a lathe, craftsman"
"divine protection"
"son of the red warrior"
"wolf counsel"
"from Percy (Norman place name)"
"gentle breeze"
"ruler of the sea"
"peace, gentleness"
"God has remembered, remembered by God"
"small sheltered coastal bay"
"from the isle"
"crooked enclosure"
"messenger (angelos)"
Similar sound: These names share phonetic patterns with Duncan — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Scottish Gaelic origins, giving them the same cultural heritage as Duncan.
Rhythm: Duncan is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.