Stalagtites can be duped by repeatedly placing a 3 tall stalagtite and breaking it. Not much else to say about it, see attached video for a demo.
Environment
Windows 10 2004
Java 1.8u51 x64 (default JRE packaged with the game)
MC-206680
Hanging dripstone stalagtite dupe
Resolved
Duplicate
0
Duplicate of MC-206613