Neil Patrick Harris leaves fans speechless over same detail as he shares photo of twins starting high school

Neil Patrick Harris has shared a sweet new picture of his teenage twins, whom he shares with his husband David Burtka - and fans are obsessing over one detail.

The How I Met Your Mother actor, 52, married David, 50, back in 2014 after first debuting their relationship in 2007.

In 2010, they became parents to twins Gideon and Harper, who are 14 years old, after a surrogate carried and gave birth to them.

And now, fans of the couple can't get over the fact that the twins are already going to high school, joking in the comments that they 'feel old' and 'I’m pretty sure they were just toddlers a few weeks ago, so there’s no way'.

Sharing a snap of the duo looking up over their laptop screens, Harris wrote in the comments: "The kids are entering high school this year (how wait what??), and they are at last allowed to rid themselves of their Chromebook anchors and ascend to the world of the MacBook. Huzzah!"

The actor has delighted fans with a new snap of his teenage twins (Instagram/@nph)

Just a few weeks before, the American Horror Story star shared a heartwarming family photo of all four of them, alongside the caption: "The loves of my life."

As social media users are being treated to more photos of the group as the twins are getting older, they seem to think one of them is striking quite the resemblance to Harris.

The children are non-identical, as two separate eggs were used, however, social media users can't get over how much they resemble each other and their dads.

One Instagram user wrote: "Aw, they’re adults now I knew Gideon would look like you but wow. Beautiful preteen adults."

While a second added: "No doubt what so ever whose kid is whose biologically lol".

However, another fan disagreed, penning: "Oh I disagree I think she’s Neil’s & he’s David’s lol," and a third quipped: "I agree! The boy looks just like David!!"

"Gideon looks more like David, his sister looks more like NPH," wrote someone else.

While others simply can't tell, writing: "How is it that they honestly look like both of you?! They're gorgeous," and "These two are so similar to both, it’s impossible to tell who’s biologically who’s. That’s so cool."

As Gideon and Harper are getting older, some fans have guessed who is biologically who's (Instagram/@nph)

Harris and Burtka each donated sperm in the hopes that either would fertilise the egg, but to their surprise and joy, the twins were conceived.

Speaking to American journalist Barbara Walters in 2014, Harris recalled: "We inserted one of my sperm and one of David's sperm into two eggs with the hope that they would both take, just because we both wanted to be dads biologically. Both took, miraculously."

He also made sure to outline that he has no interest in finding out which of his twins are his biologically.

The star explained: "I have no interest in [finding out]. We are their parents and I love them implicitly."

And while the couple have no idea about the biological element, Harris has previously admitted they have suspicions 'based on their behaviour' - however, they haven't revealed what these were.

Not that anyone needs to know - as the actor pointed out, they both belong to two loving dads and that's all that matters!