Minecraft dupilicate/glitch?
Sounds like the ghost blocks, a common Minecraft glitch.
What They Are
Ghost blocks are blocks that are not supposed to be in a certain position, but they are. They occur when the client sees a block in a certain position, but the game does not. The ghost block that results can have interesting properties.
How To Get Rid of Them
There are a few ways to get rid of ghost blocks. The first involves right-clicking on the ghost block. If the player is holding a block, then the ghost block will be replaced by the block the player is holding.
The second solution, which is much simpler, is to reload the chunks. This can be done with F3+A, moving, or logging out and logging back in.