Just because Khloe Kardashian is back with Tristan Thompson doesn't undo his past.

One woman insists that he is her Baby Daddy ... and a lot of people say that the resemblance is clear.

Kimberly Alexander took to Instagram to share a short video of her with her 5-year-old son.

She claims that Tristan Thompson is the father -- and a deadbeat one at that.

Followers, and others who saw the video, flooded the comments with agreements that Tristan "must" be the baby daddy.

“He looks like his sister True," one commenter declares, comparing this sweet little boy to Khloe's daughter.

Another remarked: “Got damn this is @realtristan13," tagging the alleged baby daddy.

Of course, paternity is not determined in the court of public opinion.

Back in May, after weeks of threats to sue, Tristan filed suit in Los Angeles court against Kimberly.

He had repeatedly appealed to her to stop claiming that he was the father of her son.

Tristan had also objected to his characterization as a deadbeat dad who was not meeting his financial obligations.

"Why on the next day file a default??" Kimberly remarked in a recent Instagram post.

"Seems to me that if u ain’t got s--t to hide," she wrote, "just take a second dna that I choose I can’t do anything to the test so why deny all my picks?"

Kimberly added: "All the back and forth was what got me dropped oh and talking out on Ig out of frustration of Lies"

What she is alluding to there is that, in court, Tristan claims to have already taken a DNA test that disproves paternity.

However, Kimberly has objected to the specific testing service, which she believes was not "neutral."

In particular, she accuses the service of having ties to the Kardashians, suggesting that it would be easy for them to "fix" a test.

In answer, Tristan's team has said that Kimberly has ulterior motives for claiming that Tristan is her baby daddy.

Specifically, his attorney notes that she has had financial difficulties, including a history of evictions.

Tristan's attorney also argues that it is in some ways suspicious that she did not immediately claim paternity, instead waiting a few years.

"Today is your 5th Birthday," Kimberly gushed in a post written on July 6 and directed at her precious son.

"Happy Birthday my sweet sweet son!" she wrote. "Every year on your birthday, I look back at all of the photos I have of you, and I cry."

"Some are sad tears because you are growing up entirely way too fast and it breaks my heart," Kimberly admitted. 

She expressed: "I wish I could keep my tiny baby little forever. However, the tears are mostly happy tears."

"I am just so thankful that God blessed me to be your mom," Kimberly remarked.

"Somehow, in the blink of an eye, you have gone from a 6lb 3 oz newborn to a 5 year old boy," Kimberly concluded. I love watching you grow! Happy birthday."

We here at THG are not expert biologists, but we know that facial features and perceived similarities do not always mean that people are closely related.

After all, we have all seen those "celebrity doppleganger" posts where people who are not related at all look jarringly alike.

That said ... sometimes, celebrities do father children and then go above and beyond to fight any claims of paternity, even if they have not done so with other children.

So how can we know the truth? Well ... we probably can't.

On the one hand, a second DNA test at a court-determined facility might be more conclusive.

At the same time, though, we can understand why any celebrity might resent the idea that anyone at all could come out of the woodwork and force them to undergo not one, but multiple DNA tests.