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"
"James reversed — an ananym honoring the name James"
"sky father, divine sky"
"joy, laughter"
"son of Cole"
"warlike, martial"
"will, desire, helmet"
"power of the tribe"
"bold counsel, brave advisor"
"son of the prince/lord"
"born of fire"
"young warrior, free man"
"strong, upright"
"just, righteous, equitable"
"beloved of God"
"loud revelry"
"raven-like, dark as a raven"
"strength, permanence"
"noble eagle (adal + aro)"
"defender of men"
"elf warrior"
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.