RPM Collection Agency illegal immoral statute of limitations lynnwood, Washington

This company is out of control.  They are calling me about a bill I know was paid over 15 years ago.  I have no record now, as the bill is over 15 years old!  The date they gave me was 9-18-94!  They keep calling my home. 

They are at an unfair and immoral advantage as they claim they have the ability to report to the credit bureau and I have no paper record that I paid it.  I am sure most people do not keep records/ receipts that date that far back.  I am not trying to avoid paying a bill, but avoid paying it twice!  I pay my bills and on time. 

I had to ask several times just to get an address from them, yet they wanted my social security number. I don't care if they want $1.  This is completely ridiculous.  I am being targeted for a credit mistake 15 years ago.  Is this even legal.  There should be a statute of limitations and they should be required to give more information.  

They told me their company just bought the debt and that is why they are calling me now.  How is it legal for these companies to keep buying each other out and have records obviously getting lost in the process.  I know I paid this1  As I remember it haunting me 15 years ago! 

Jodi