一つ（ひとつ）— one (general counter)
パン を 一つ 買います。
I’ll buy one piece of bread.
一つひとつ
one (general counter)
ひとつ
accent [2]In detail
Words
パンpanbread
をo(object marker)
一つhitotsuone item
買いますkaimasubuy (polite)
Kanji
一イチ・
一つ is read ひとつ and counts one item. The ～つ series works for many things, but Japanese also has more specific counters; it is not a replacement for every counter.
Coco-sensei