How can I set id of a component/tag inside ui:repeat

This is not possible with a render-time tag such as <ui:repeat>. The <ui:repeat> will however by itself already ensure the uniqueness of the generated client ID by prepending it with the row index. So just remove the EL part from the ID attribute of the component.

<ui:repeat value="#{bean.columns}" var="column">
    <h:panelGroup layout="block" id="column">

With a view build time tag such as <c:forEach> (which will basically generate multiple <h:panelGroup> components instead of only one which is rendered multiple times), it is possible to specify a dynamic ID like that.

<c:forEach items="#{bean.columns}" var="column">
    <h:panelGroup layout="block" id="column_#{column.id}">

(you should only be well aware of how JSTL works in Facelets)

An alternative is to use a static <div> element instead of a JSF <h:panelGroup layout="block"> component.

<ui:repeat value="#{bean.columns}" var="column">
    <div id="column_#{column.id}">

See also:

• JSTL in JSF2 Facelets... makes sense?

JSF prefixes the id automatically. If you simply write id="column" the generated HTML will contain such identifiers:

myForm:0:column myForm:1:column myForm:2:column

and so on.

Anyway: Do never use JSTL tags (like c:foreach and c:if) in JSF templates. They cause random behaviour, very difficult to debug. And if they work, the slow down the application a lot.

Use ui:repeat for loops, and ui:fragment for conditional blocks. Note that there is no replacement for c:set, such a construct does not exist anymore in JSF 2.