T. duplicatus group
This group contains all the Toxomerus with simple patterns of horizontal bars on the abdomen and usually a pair of commas on each abdomen section. Common species in this group are likely T. duplicatus and T. watsoni, however there are many other species with a similar pattern.

"Many species of Toxomerus present the same abdomen color pattern as Toxomerus duplicatus and the morphology of the male and female genitalia ... helps their segregation." - Borges & Couri 2009, pg. 32

Note that in some locations, such as Florida, observations may be identifiable to species by range.

Possible list of species that resemble this:

T. brevifacies
T. buscki
T. corbis/planiventris?
T. difficilis
T. duplicatus
T. ecuadoreus
T. idalius
T. minutus?
T. norma
T. papaveroi
T. paraduplicatus
T. pinchinchae
T. productus
T. puellus
T. steatogaster
T. taenius
T. watsoni

(draft, please let me know of any more or of any way to identify particular species)

T. floralis/dispar
These two species are both very variable and also widely ranging and share a range in many areas in Central and South America, as well as Florida. See Thompson 1981 (Flower Flies of the West Indies) for the most details about this pair.

https://www.inaturalist.org/observations/6744479

T. cf. aquilinus "group"
A group of 4 very similar species from South America.

"Toxomerus aquilinus, intermedius, teliger and tibicen have been frequently confused given the identifications we have seen in collections. All these species appear similar. Of the four, aquilinus is readily distinguished by the black subapical fasciae on the 3rd and 4th terga being nearly straight so that the lateral margin on the basal 1/2 is pale (Fig. 4). The other species have the black fasciae extending anterolaterally so that the lateral margin is black along most of its length (Figs 2, 8)." - Metz & Thompson, pg. 234

"Toxomerus intermedius and teliger are sister species. They share a unique shape and setal pattern of the paramere, which is not found elsewhere among Toxomerus species. They differ mainly in wing microtrichia (alula bare in teliger, microtrichose in intermedius) and male genitalia (dorsal arm of paramere with 4 long setae in teliger, two in intermedius), Toxomerus intermedius has been rarely collected, being known only from six collections." - Metz & Thompson, pg. 238

"Toxomerus teliger looks like aquilinus, but is separated from all other large species by its bare alula" - pg. 247

"Toxomerus tibicen is similar to aquilinus, intermedius and teliger in appearance but as discussed under those species it can be readily distinguished by its abdominal pattern and microtrichose alula" - pg. 249