Is there ayone else besides Lemaitre who has also been overlooked? The answer may well be yes - Hermann Weyl. In an appendix to the fifth edition of his book Raum, Zeit, Materie (1923) Weyl presented an equation that, to first order, is the Hubble relation. He also made a crude estimate of the Hubble radius. In a parallel paper (1923, Phys. Zeitschr, 24, 230) he presented the full derivation of the Hubble relation, starting with an expanding model of the universe. In other words, Weyl made essentially the same calculations as Lemaitre, and he did so four years earlier. Shouldn't he receive credit?
Arguably the answer is yes, particularly since Lemaitre actually cited Weyl's paper in both the French and English versions of his paper and thus was aware of Weyl's derivation. Robertson (1933 Rev. Mod. Phys., 5, 62, p. 88) certainly gave him credit (Bibliography): "1923 H. Weyl ... Introduction of postulate concerning coherency of matter and deduction of linear velocity-distance relationship." Robertson also referenced Lemaitre's 1927 paper but made no mention of any contribution by him to the v-D relationship.
Regardless, it is interesting to compare the details of what Weyl and Lemaitre actually did.
In his book, Weyl made a crude calculation of the Hubble radius, which, for de Sitter space, is the same as the 4-d radius of curvature of that space. He used Lundmark's estimated distance to Andromeda (1 to 10 million light years), assigned a velocity of 1000 km/s (a typical value measured by Slipher), and combined them to get a radius of 10^9 light years. If we convert that to a Hubble constant, we get 978 km/s/Mpc. (Weyl next computed the ratio of the Hubble radius to the electron radius and thus entered the world of the Large Number Hypothesis.) Weyl's estimate is obviously of limited value - while the distance to M31 was quite reasonable, the velocity he used was an artificial assignment based on other galaxies. However, we have to remember the state of the field at this time. Distances to the spiral nebulae were still unknown in general. Hubble's Cepheid distance work was still in the future. Lundmark's distance was based on novae and Opik's method, both of which were considered speculative at the time.
How does Lemaitre's work compare with Weyl's? On the theory side, Lemaitre took a big step forward by incorporating matter along with the cosmological constant. As far as the Hubble law, however, a linear relation between distance and velocity is a generic feature of any model of a homogeneous, expanding universe, so in that respect, Lemaitre added little to what Weyl had already shown. The Hubble constant itself now depended on two parameters of the theory (density of matter and lambda) instead of one, but because dark energy was dominant, Lemaitre gave an approximate equation for the Hubble constant that was identical to Weyl's.
On the observation side, Lemaitre also took a big step forward, but that is because the field itself had advanced and nearly caught up to the theory. In particular, Hubble's 1926 paper (ApJ., 64, 321), with apparent magnitudes and distance calibration, had been published. (Lemaitre also made use of Stromberg's 1925 paper, ApJ., 61, 353, which compiled a list of positions and velocities of galaxies, although nearly the same information was already available in Eddington's book. Lundmark's 1924, MNRAS, 84, 747 and 1925, MNRAS, 85, 865 papers deserve mention as well.) Hubble's paper, in particular, was crucial to Lemaitre. First, Hubble provided the distances to 7 local group galaxies; Weyl only had a distance to M31. Second, for more distant galaxies, in which Cepheids could not be detected, Hubble gave a tentative equation to convert apparent magnitude to distance (assuming a galaxy can be treated as a standard candle.) At best, Weyl could have used Andromeda alone as a standard candle (which is what Lundmark did, or at least, tried to do, in 1924.) Third, Hubble provided a convenient table where he assembled data for a large number of galaxies, including apparent magnitudes. (Lundmark was the first to rely on the works of Holetschek and Hopmann for magnitudes, which Hubble used as well, but Lundmark did not publish the actual magnitudes that he used.) Fourth, Hubble estimated the mean density of matter in the universe, and Lemaitre used it as one of two observational quantities needed to constrain the free parameters of his model (the other being the Hubble constant itself.) By combining the distances and magnitude data from Hubble's paper with the velocity and position data (the latter being needed for the vertex motion calculation) compiled by Stromberg, Lemaitre's task was greatly simplified.
The real accomplishment of Lemaitre was that of taking disparate pieces of work - even if derived previously - and assembling them into a complete, self-consistent, evolving model of the universe that, for the first time, was matched to observations of both the density of matter and the rate of expansion. Further, unlike Weyl (1930, Phil. Mag., 9, 936), who considered his explanation of Hubble's "strange phenomenon" to be "speculative", Lemaitre was unequivocal in asserting that an expanding universe was the correct explanation.