It means that coincidences happen; not only one but a pair of similar things occur together.
独一个；偶一双。不只一个，竟然还有配对的。表示两事或两人十分相似。
刘半农《奉答王敬轩先生》先生与这位老夫子，可称无独有偶。”
The two scientists independently made the same discovery on the same day—truly, no single occurrence without a pair.
Learn the next connected idiom: 无独有偶 (wú dú yǒu ǒu) → 偶一为之 (ǒu yī wéi zhī)
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