From their first appearance in Tintin in the Congo, the Thom(p)sons have been hard to tell apart.

Even Benoît Peeters got it wrong in Tintin and the World of Hergé. On page 59, Peeters writes “Identical in every way, the two can only be determined by the style of their moustaches: Thomson’s is more rounded and Thompson’s more pointed.”

But as the tableau here clearly shows, Thompson, with a “p,” has the rounded mustache and Thomson, without a “p,” has a pointy mustache.

The two detectives were originally called X-33 and X-33A, and later named Dupont and Dumond in French.

Thompson and Thomson appear in only one frame of The Shooting Star, on page 20.

Thompson and Thomson’s first appearance, in Tintin in the Congo. Only a slight difference in mustaches is visible.

In The Black Island, page 2, there’s a clear mustache difference.

In The Seven Crystal Balls, page 18

In The Seven Crystal Balls, page 24

In The Seven Crystal Balls, page 48

The Red Sea Sharks, page 7, frames 12 and 13

Tintin and the Picaros, page 60 frame 2.

The Castafiore Emerald, page 47, frame 2

The Castafiore Emerald, page 60 frame 11.

It’s as clear as day.
To be precise: Unless the Thom(p)sons themselves are consistently confused, Thomson, on the left below, has the pointy mustache, Thompson, on the right below, has the droopy mustache.
That’s our opinion and we’re stuck with it.