This is an odd one, but I think you're right in thinking it's an error, I doubt only the Saloon version of the Mondeo would have two engines that were 2.5l. I did some research anyway and I can't really find any actual evidence there were two versions of the same engine in the MKIII. But, notably they aren't technically the same engine if you go by capacity. The two "variants" (as listed on eBay for example) are distinct as the one that marks itself out as the 24V "variant" is a 2495cc engine, the same for all 2.5l V6 engines used in not just the Saloon but Hatchback and Estate MKIII's too, and the non 24V version is listed as 2451cc. That's why it's odd, because I can't find any evidence anywhere of a 2451cc version of the Duratec ever existing.
History time! The Duratec 2.5l being used in as far back as the first generation, pre facelift (i.e. MKI) Mondeo was a 2544cc engine, introduced in 1994 on Ghia, Ghia X and SI trims of the MKI - they were badged as 24V originally since Ford didn't start using the V6 badge until I believe the MKII. With the MKII, they carried the engine over, but also stuck it in the new ST24 as well as the Ghia/Ghia X/SI trims, and later also used it in the ST200. The engine used was still 2544cc at this point.
When Ford released the MKIII in October 2000, the Duratec 2.5l was carried over but made to be slightly smaller capacity at 2495cc, which I believe is what Alpid above was talking about. Thing is he makes it sound like there were two versions of the Duratec 2.5l in the MKIII but this was not the case; he's referencing the fact the MKII 2.5l v6 was 2544cc where as when it got transplanted into the MKIII they removed the EGR valve and among other minor alterations also made the capacity of the engine slightly smaller at 2495cc.
So, in closing where this 2451cc engine has come from, I don't know. No one probably does except the muppet who first put it into the database all these websites use. There are only two possible explanations; most likely it is an error as you thought that was made at some point in the last 20 years which has resulted in a national database of car makes and models showing an extra engine for the MKIII Mondeo Saloon that doesn't exist. Or, for some reason Ford experimented with a 2451cc engine in the very earliest MKIII 2.5l v6 Saloons, or maybe even with a prototype that was registered as road legal but never went into production which wound up getting listed on the database.
That last one is about as likely as the second coming happening if you ask me. It's an error, I'm pretty damn sure.