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This error appears when you have at least one label element in your form that invalidly contains two or more form controls. A label must be corresponded to a single form control.

This is often caused by wrapping checkboxes or radio buttons with <label></label>, such as in the example below:

<label>[checkbox your-country "China" "India" "San Marino"]</label>

Contact Form 7 provides a use_label_element option for labeling checkboxes and radio buttons. Use this instead of label elements.

Contact Form 7 4.5’s release announcement describes the story behind the introduction of this validation.