Confusing Issue Possibly Related To onMouseDown()

I have a class called GameBlock. Basically, I want the player to be able to click on two blocks, and the blocks swap positions with each other. Simple in theory, but I'm having a bit of trouble with the execution.

I have three relevant functions here: onMouseDown(), onMouseUp(), and rotateBlocks(). The onMouseUp() was an afterthought once I encountered this problem, and to be frank, it hasn't made a difference. I think the core of my problem lies in the fact that onMouseDown() seems to get called twice, for every click of the mouse. (This is revealed in the echo statements of rotateBlocks().

function GameBlock::onMouseDown(%this, %modifier, %worldPosition, %clicks)
{
    $mouseButtonClicked = true;

    if(!$firstBlockSelected)
    {
        $firstBlockSelected = true;
        %this.blockSelected = true;
        $startingBlock = %this;
    }
    else if(%this.blockSelected)
    {
        %this.blockSelected = false;
    }
    else
    {
        $lastBlockSelected = true;
        %this.blockSelected = true;
    }

    if($firstBlockSelected && $lastBlockSelected)
    {
        for(%i = 1; %i < 101; %i++)
        {
            if($GameBlock[%i].blockSelected == true)
            {
                $finishingBlock = %this;
                $firstBlockSelected = false;
                $lastBlockSelected = false;
                rotateBlocks();
            }
        }
    }
}

function GameBlock::onMouseUp(%this, %modifier, %worldPosition, %clicks)
{
    $mouseButtonClicked = false;
}

function rotateBlocks()
{
    if($mouseButtonClicked)
    {
        // Get the initial positions of the first and last blocks
        %startingBlockPositionX = $startingBlock.getPositionX();
        echo("Starting Block Position X:" SPC %startingBlockPositionX);
        %startingBlockPositionY = $startingBlock.getPositionY();
        echo("Starting Block Position Y:" SPC %startingBlockPositionY);

        %finishingBlockPositionX = $finishingBlock.getPositionX();
        echo("Finishing Block Position X:" SPC %finishingBlockPositionX);
        %finishingBlockPositionY = $finishingBlock.getPositionY();
        echo("Finishing Block Position Y:" SPC %finishingBlockPositionY);

        // Order the blocks to swap places
        $startingBlock.setPositionX(%finishingBlockPositionX);
        $startingBlock.setPositionY(%finishingBlockPositionY);
        $finishingBlock.setPositionX(%startingBlockPositionX);
        $finishingBlock.setPositionY(%startingBlockPositionY);
    }
}

Now here is the confusing part. No blocks are visibly moved, but the echo commands swap the positions around. For example, an output I might get after clicking two blocks (and thus a swap should occur) is this:
Starting Block Position X: -12.35
Starting Block Position Y: -17.35
Finishing Block Position X: -7.35
Finishing Block Position Y: -12.35
Starting Block Position X: -7.35
Starting Block Position Y: -12.35
Finishing Block Position X: -12.35
Finishing Block Position Y: -17.35

But no blocks move. However, if I comment out one pair of setPosition commands and only have...

$startingBlock.setPositionX(%finishingBlockPositionX);
        $startingBlock.setPositionY(%finishingBlockPositionY);

...or...

$finishingBlock.setPositionX(%startingBlockPositionX);
        $finishingBlock.setPositionY(%startingBlockPositionY);

...then one block will move to the correct position, and the other, predictably, stays put. So why is it that I tell both blocks to move that nothing visibly happens?