HTML caching is arguably the best way to improve Sitecore performance. Sometimes you can run into issues when you enable HTML cache on a rendering and the rendering has been added to the same page multiple times. This will only happen if the renderings do not have a datasource or share the same datasource, but will still render different content. This could happen for example when the renderings have different rendering parameters or have some custom logic which changes the content.

This can be fixed in a generic way by overriding the GenerateKey method of the GenerateCacheKey RenderRendering Processor. Below code will add the UniqueId of each rendering to the cachekey which will ensure the cached output is unique for each rendering.

using Sitecore.Mvc.Pipelines.Response.RenderRendering;
using Sitecore.Mvc.Presentation;

namespace Foundation.Pipelines.RenderRendering
{
    public class GenerateCustomCacheKey : GenerateCacheKey
    {
        protected override string GenerateKey(Rendering rendering, RenderRenderingArgs args)
        {
            var cacheKey = base.GenerateKey(rendering, args);

            cacheKey += rendering.UniqueId;

            return cacheKey;
        }
    }
}