For your repeat it works exactly the same, you use the static id for the expressions inside the current repeat item and the dynamic id to have unique ids in the DOM.
Here your sample code with a repeater
<html>
<head>
<meta name="ac:base" content="/daviesandgray.co.uk/httpdocs/httpdocs">
<base href="/daviesandgray.co.uk/httpdocs/httpdocs/admin/">
<meta charset="UTF-8">
<title>Untitled Document</title>
<script src="../dmxAppConnect/dmxAppConnect.js"></script>
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<link rel="stylesheet" href="https://pro.fontawesome.com/releases/v5.10.1/css/all.css" integrity="sha384-y++enYq9sdV7msNmXr08kJdkX4zEI1gMjjkw0l9ttOepH7fMdhb7CePwuRQCfwCr" crossorigin="anonymous">
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.2/moment.min.js" integrity="sha256-CutOzxCRucUsn6C6TcEYsauvvYilEniTXldPa6/wu0k=" crossorigin="anonymous"></script>
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
<script src="../dmxAppConnect/dmxBootstrap4Modal/dmxBootstrap4Modal.js" defer=""></script>
<script src="../dmxAppConnect/dmxFormatter/dmxFormatter.js" defer=""></script>
<script src="../dmxAppConnect/dmxStateManagement/dmxStateManagement.js" defer=""></script>
</head>
<body is="dmx-app" id="test1">
<dmx-local-manager id="local1"></dmx-local-manager>
<div class="container">
<div class="row">
<div class="col">
<div class="row">
<div class="col">
<button class="btn btn-info" dmx-on:click="modal1.show()">Button</button>
</div>
</div>
<div class="modal" id="modal1" is="dmx-bs4-modal" tabindex="-1" role="dialog">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
<h5 class="modal-title">Modal title</h5>
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="modal-body">
<p>Modal body text goes here.</p>
<form id="form2" is="dmx-serverconnect-form" method="post">
<dmx-value id="var_id_number1" dmx-bind:value="'a1_id_125_1'"></dmx-value>
<dmx-value id="var_id_number2" dmx-bind:value="'a2_id_125_1'"></dmx-value>
<dmx-value id="var_id_group" dmx-bind:value="'input2_group'"></dmx-value>
<div is="dmx-repeat" dmx-bind:repeat="5">
<div class="form-group" id="input2_group" is="dmx-checkbox-group">
<div class="form-check form-check-inline">
<label class="btn btn-xs btn-outline-success" dmx-bind:for="a1_id_{{$index}}" dmx-class:active="a1_id_125_0.checked">
<input class="custom-control-input" type="checkbox" value="Door" id="a1_id_125_0" dmx-bind:id="a1_id_{{$index}}" name="paint[]" dmx-on:change="a2_id_125_0.select(false)">
paint a1_id_{{$index}}
</label>
</div>
<div class="form-check form-check-inline">or</div>
<div class="form-check form-check-inline">
<label class="btn btn-xs btn-outline-success" dmx-bind:for="a2_id_{{$index}}" dmx-class:active="a2_id_125_0.checked">
<input class="custom-control-input" type="checkbox" value="Door" id="a2_id_125_0" dmx-bind:id="a2_id_{{$index}}" name="paint[]" dmx-on:change="a1_id_125_0.select(false)">
blend a2_id_{{$index}}
</label>
</div>
</div>
</div>
</form>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>
<button type="button" class="btn btn-primary">Save changes</button>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
</body>
</html>