dotcommadotcomma, 48 bytes
[[],][.[[[,],],]][.[,]][,[,[,.[[[,][,]][,.].]]]]
<script src="https://combinatronics.com/RedwolfProgramsRadvylf/dotcomma/master/interpreter.js"></script><script src="https://code.jquery.com/jquery-3.5.1.min.js"></script><script>$(document).ready(function () {$("#btnInterpret").click(function () {$("#txtResult").text(interpret($("#txtCode").val(), $("#txtInput").val(), $("#lstOutputAs").children("option:selected").val()));});});</script><style>.textBox {background-color: white;border: 1px solid black;font-family: Courier New, Courier, monospace;width: 100%;}</style>Code: <textarea id="txtCode" type="text" class="textBox" style="height: 200px">[[],][.[[[,],],]][.[,]][,[,[,.[[[,][,]][,.].]]]]</textarea><br />Input: <textarea id="txtInput" type="text" class="textBox">Double speak!</textarea><br /><input id="btnInterpret" type="button" value="Run" />Output as: <select id="lstOutputAs"><option value="true">String</option><option value="">Number array</option></select><br />Result:<br /><div id="txtResult" class="textBox" style="overflow-wrap: break-word"></div>Code:
[[],] insert end marker (0) [.[ while characters left [[,],], copy each character 3 times ]] [.[,]] go to first 0 [,[,[, delete copies of 0 and first character .[[[,][,]][,.].] delete every third character until we reach 0 again ]]]
