TYPO3 Core

There are scenarios in which the same form is rendered several times even though it is only output once on the page.
Assuming there is a form plugin with a form with validators on form elements on colPos 10, the following setup would render the plugin twice and output the second rendered plugin.

setup.typoscript

page = PAGE
page {
    10 = FLUIDTEMPLATE
    10 {
        file = EXT:site_package/Resources/Private/Templates/Index.html

        variables {
            content-col-10 < styles.content.get
            content-col-10.select.where = colPos=10
        }
    }
}

lib.getSomeColPos = CONTENT
lib.getSomeColPos {
    table = tt_content
    select.where = colPos=10
    select.orderBy = sorting
}

EXT:site_package/Resources/Private/Templates/Index.html

<f:cObject typoscriptObjectPath="lib.getSomeColPos"/>

The result is that the validators are missing in the 2nd rendered form.
This is because when the validators are added, a runtime cache is used to prevent validators from being added twice (needed due to internal processes).
When rendering the same 2. form no more validators are added because the form element is not identified clearly enough.

A simple test to fake such a scenario without much effort would be:

setup.typoscript

page = PAGE
page {
    10 = FLUIDTEMPLATE
    10 {
        file = EXT:site_package/Resources/Private/Templates/Index.html

        variables {
            content < styles.content.get
            content-foo < styles.content.get
        }
    }
}

EXT:site_package/Resources/Private/Templates/Index.html

{content-foo -> f:format.raw()}

or

setup.typoscript

page = PAGE
page {
    10 = FLUIDTEMPLATE
    10 {
        file = EXT:site_package/Resources/Private/Templates/CustomContentElement.html
        settings {
            persistenceIdentifier = 1:/form_definitions/test.form.yaml
        }
        extbase.pluginName = Form
        extbase.controllerExtensionName = Formframework
        extbase.controllerName = FormFrontend
        extbase.controllerActionName = perform
    }
}

EXT:site_package/Resources/Private/Templates/CustomContentElement.html

<formvh:render persistenceIdentifier="{settings.persistenceIdentifier}" />
<formvh:render persistenceIdentifier="{settings.persistenceIdentifier}" />

1:/form_definitions/test.form.yaml

type: Form
identifier: test
label: test
prototypeName: standard
renderables:
  -
    type: Page
    identifier: page-1
    label: Step
    renderables:
      -
        defaultValue: ''
        type: Text
        identifier: text-1
        label: Text 1
      -
        defaultValue: ''
        type: Text
        identifier: text-2
        label: Text 2
        properties:
          fluidAdditionalAttributes:
            required: required
        validators:
          -
            identifier: NotEmpty
      -
        defaultValue: ''
        type: Text
        identifier: text-3
        label: Text 3
        properties:
          fluidAdditionalAttributes:
            required: required
        validators:
          -
            identifier: NotEmpty
  -
    type: SummaryPage
    identifier: summarypage-1
    label: 'Summary step'

result: