noun
the same thing; one single thing
Refers to something that is identical or limited to one item; often used in contexts emphasizing repetition or singularity.
彼はいつも一つ事ばかり言っている。
He always says the same thing over and over.
一つ事にこだわりすぎないほうがいい。
It's better not to obsess too much over one single thing.
同じこと is more common for 'the same thing' in general; 一つ事 often carries a nuance of 'one and the same' or 'just one thing'.
Compound of 一つ (one) and 事 (thing, matter). The exact historical derivation is uncertain, but the meaning is transparent from its components.