Couldn't find a duplicate for this.
When placing blocks (especially half-slabs; occurs with other blocks as well however), the game can sometimes double-input and place two.
I've found this to occur when using both gamepad and keyboard/mouse input methods. I made sure it wasn't my input peripherals (mouse, keyboard, gamepad; drivers reinstalled, tried with other variants such as Xbox 360 controller vs. Xbox One controller, etc).
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[Mojang] Mega_Spud (Jay)
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-26819.
Your additional information and observations are very useful, so if you can add them to the comments section of the parent issue it would be appreciated.
If you have not, you might like to make use of the search feature before making a new ticket, as it's likely that the issue has been reported already.
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I just purchased the brand new GameSir Galileo G8 Plus Bluetooth controller that supports newer Bluetooth technologies and I am STILL suffering from double inputs during gameplay. This negativity impacts quick controls that could result in the death of the player, such as failing to draw your bow when fighting a Ghast, bridging over the void in The End, or even mining for netherite using beds in The Nether. This happens with every Bluetooth controller I use, less likely with my Xbox Elite 2 Controller. I play on Bedrock on Android, using the most recent version, on device and on realms, on 5G or WIFI.
They are saying that this too-fast block placement bug is a duplicate of this: https://bugs.mojang.com/browse/MCPE-26812
Not sure I agree, since a mod changed the description of that other bug to be about opening/closing/using objects, not about placing blocks. And recent reports seem to show they've fixed the opening/closing/using problem, but not the block placement problem.