When placing a candle on a cake, then shift-right clicking on a candle, it leads to a weird-looking double candle tower.
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Hello! This looks to be a duplicate of MC-203572.

The behavior of candles in this snapshot is admittedly a bit strange. They act like actual blocks, although they shouldn't. I think they should hopefully end up being like saplings or flowers, where it is not possible to place anymore of the same block on top of itself. I attempted to put saplings on top of saplings, and flowers on top of flowers, and of course this does not work, and I think that candles should be the same way.