Especially season 4 Sam Winchester and Doug have some similarities in their plotlines

Doug is a pastor and Sam prays and believes. Has faith, or whatever.

Sam's demon blood arc in season four. / Doug turning into a dinosaur

There is evil inside of Sam (that's how they see it) but Sam tries to turn it into something good (kills demons with it). DOUG turns into a dinosaur and is a bit shocked and scared but he also tries to turn it into a good thing (kills ninjas with it)

THEY BOTH HAVE SOMETHING EVIL INSIDE OF THEM AND THEY'RE SCARED OF IT AND IT IS UNGODLY. but also. Gives them control. (Doug kills Frankie mermaid because Frankie mermaid killed his parents)

Speaking of. It sort of starts to bleed into their personalities. Doug kills Frankie mermaid even though he's not really a dinosaur then. Sam is more ruthless than normally the whole demon blood arc.

Both Doug and Sam feel like they're different. Othered from the world. "Like, really different" or whatever they said in VelociPastor (queer coding?)

Ruby/Carol. Both encourage Sam/Doug to use this violent thing inside of them to kill bad guys, they say it doesn't have to be a bad thing

(except that ruby is actually just manipulating Sam. Which I support, by the way. I love Ruby.)

Dean/father Steward does Not like this power. Is kind of afraid of it.

Dean says that Ruby is manipulating Sam (demon blood?? Hello??? Sin.). Father Stweard says that Carol is converting Doug into sin.

The double life with Sam at the beginning of season 4. Sam kills demons with Ruby at night and hides it from Dean. / Doug kills ninjas in secret at night with Carol and doesn't tell anyone about it.