This bugpost is not about forcing success if you /setblock or /fill an already-existing block( s ), but about changing the somewhat imprecise error message "The block couldn't be placed." (Since 17w46a: ) "Could not set the block.".
Furthermore, the message is inconsistent with the one for /fill in a similar setup: "No blocks filled".

As by now also the error messages for commands improved greatly, I feel the error message for this case should also be changed, as the output is "too vague".

Test setups for /setblock

If I try to set an air block where there is nothing, basically "already air", it gets me the error message "The Block couldn't be placed." (Since 17w46a: ) "Could not set the block."

/setblock ~ ~ ~ minecraft:air

The same also goes for e.g. a naturally spawned white/yellow sandblock, if you stand on it and do

/setblock ~ ~-1 ~ minecraft:sand

and even if you insert its (currently still in the game) datavalue like so:

/setblock ~ ~-1 ~ minecraft:sand 0

then it still gives you that error message.

Wouldn't it make more sense/be more precise to output an error message along the lines of e.g.:
"Same block already at destination" or "No block changed" or similar?

Inconsistency with error message for /fill

If you try to /fill an area that you already filled before with stone, again with stone, you get at least the error message displayed in the command block: "No blocks filled."

If you try to fill just a single block that you already filled before with stone, again with stone, you get a different error message displayed in chat than the /setblock-error message: "No blocks filled."

This bugpost is not about forcing success if you /setblock or /fill an already-existing block( s ), but about changing the somewhat misleading or inconsistent error message "The block couldn't be placed.". It would be maybe better for both beginners in CBing, as well as advanced CBers for debugging their maps/contraptions, if the (error) message would be more precise, like suggested above, and maybe also consistent with the /fill-message, if the Devs think this would be better for CBers.