Recently I came across this odd question. I hope, Serial number is unique across certificates signed by a particular CA. So is there any chance that two different CAs sign two different certificates with same serial number?

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    The answer to the following gives you your answer: security.stackexchange.com/questions/35691/… – R15 Dec 18 '15 at 8:09
  • So, No CA in this world creates a certificate with already used serial number right? How CAs makes sure that a serial number is not used? And Does this apply to Internal CAs as well? – rgk Dec 18 '15 at 8:18
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    Did you actually read the accepted answer? Quote: 'A CA is supposed to choose unique serial numbers, that is, unique for the CA. You cannot count on a serial number being unique worldwide' – R15 Dec 18 '15 at 8:23