not alone but in pairs (idiom, usually derog.); not a unique occurrence
it's not the only case
Definitions (粵典–words.hk)
語句
唔止一個,竟然仲要有第二個;表示兩件已經非常罕見嘅事或者兩個人非常相似 not to come singly but in pairs;
我兩個好朋友竟然喺冇夾定嘅情況之下買咗今期六合彩嘅同一set number,無獨有偶,仲要俾佢哋兩個仲埋頭獎添!
我两个好朋友竟然喺冇夹定嘅情况之下买咗今期六合彩嘅同一set number,无独有偶,仲要俾佢哋两个仲埋头奖添!
yue:我兩個好朋友竟然喺冇夾定嘅情況之下買咗今期六合彩嘅同一set number,無獨有偶,仲要俾佢哋兩個仲埋頭獎添! Two of my best friends have bought the same mark six tickets without knowing each other's number; surprisingly, they even got the 1st prize together!
Definitions (Wiktionary)
phrase, idiomatic
to coincidentally have a parallel; to happen to have a similar counterpart