Yet some things never change and Doug Ramsey, chief investment officer at Leuthold Group, has been on a mini-campaign highlighting the parallels between 2000 and 2018.

Among the numerous similarities is the elevated valuation of the S&P 500 then and now, which Ramsey illustrates in a chart that he has dubbed as the "scariest chart in our database."

"Recall that the initial visit to present levels was followed by the S&P 500's first-ever negative total return decade," he said in a recent blog post.

Price-to-sales ratio is one measure of a stocks value. It isn't as popular as the price-to-earnings ratio, or P/E, but is viewed as less susceptible to manipulation since it is based on revenue.

He also shared a chart which he claims is "unfit for a family-friendly publication" that shows how in terms of median price to sales ratio, the S&P 500 is twice as expensive as it was in 2000.