Got an invalid certificate error while accessing a website using Firefox

I was trying to access a very important website using Firefox when I got this error message about invalid certificate. I tried getting the resolution from the internet but received various ones. I then chanced upon this forum thinking that you might have the right solution.
I rely on your expertise and help will much be appreciated.

Alert
You have received an invalid certificate. Please contact the server administrator or email correspondent and give them the following information:
Your certificate contains the same serial number as another certificate issued by the certificate authority. Please get a new certificate containing a unique serial number.
