The T-34/0 keyboard layout by jonas_h features a repeater key.
KBD.news Published September 5, 2021
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The big change to the layout is the Repeat key. The concept is simple: when you press it you repeat the last pressed key – jonas_h.
I hadn't thought of that but this is an interesting concept.
Investigating the author's corpus, the Repeat could be useful, however, it does not qualify into the top 10 and so it does not outcompete Space or e as a thumb key or any frequent alphas of the home positions.
It's the 12th most frequent "letter" (without Space) so he would put it somewhere in the home box.
Felix Kühling explains how hexagonal keys work surprisingly well in an ergonomic layout without fine-tuning parameters like hand rotation, column stagger and splay.