I took a look at an online manual and it says this:
To pick up an object, click on it using the left button. The object is now illuminated
and your character is holding it. Right click to return to the game: you will see the
selected object displayed on the top right of your screen. If you wish to return this
object to the inventory, click on it in the inventory again; the object is no longer illuminated
and your character has put the object away.
Can you pick up the new blowpipe while leaving the one you don't want in your inventory? I don't know how the controls work for this game, other than what I could find in the game manual.