Is there a single or double bond between S and O in propanethial S-oxide?
The $\ce{S-O}$ bond in sulfoxides is best considered as a single covalent bond with positive charge on $\ce{S}$ and negative on oxygen. The oxygen is significantly nucleophilic (see for example the Swern oxidation mechanism 1) and the sulfur will stabilise an anion on the alpha carbon.
$\ce{C=S}$ has significant enol content and is better considered $\ce{HS-C=CR}$. The $\ce{S}$ is significantly nucleophilic and is easily oxidised to the ene-disulfide $\ce{RC=C-S-S-C=CR}$.