Report: Jessica Simpson's fiance was cheating on her -- with his estranged wife -- early in their relationship

When they first started going out, Jessica Simpson's fiance Eric Johnson couldn't make up his mind between Simpson and his soon-to-be ex-wife, so he kept sleeping with both of them, Star magazine reports.

Not only that, the tabloid says the table for such a situation was set early on as Simpson and Johnson first met when he was on a date ... with the same soon-to-be ex, freelance stylist Keri Johnson.

“He’s a two-timing cheat!” a source told the magazine. “He was still having sex with Keri after he started seeing Jessica. She needs to think twice before marrying him. He’s already cheated on her once — he’ll most likely do it again.”

Other friends say the Johnsons were already having problems with their marriage when he met Simpson, and Eric told Keri they should take a break "hoping it would ultimately make their marriage stronger,” but then immediately fell in with Simpson.

Simpson gave birth to Johnson's daughter in May, but there is still no ring on her finger.

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“It won’t be this year,” she told Katie Couric in September. “I really want to do a big, fun wedding with my friends and family.”