Feature: Make it possible to Update Fields, Content Types and Lists

Problem

The problem is: It is not possible to update site columns, content-types and lists using Invoke-PnPSiteTemplate

Story

I am frustrated when I want to update a site with new site columns, content types and Lists. If a site column already exist, Invoke-PnPSiteTemplate fails and stops.
Invoke-PnPSiteTemplate: A duplicate field name "TotalTasks" was found.

I can provision to a new site, but I need to apply it to an existing site due to keeping data and links for people.

I am working with ALM on Power Platform projects using SharePoint Lists as the storage.
On a DEV site, I create Lists holding the data.
I use Site Content Types (Not from the Content Type HUB). We do not want to mess up the HUB with our experiments.
And of course I use Site Columns (Fields) in the Content Types.

A User-Acceptance (UA) site is used to test the solutions by Users.
I provision the DEV site to the UA site. All fine the first time.
After a while users have been filling up the lists with their test data. And they have used a lot off effort getting the data into these lists.

After a while developing, new columns, content-types and lists must be added to the UA site.
And changes to existing Content-types, columns and list must also be applied.
This is ongoing work, so we want to implement it into Azure DevOps running PnP PowerShell.

Possible solution

I want to be able to use some kind of parameter to specify what I want to overwrite.
I also would like to have a filter for Fields, like: -FieldGroupsToInclude, similar to the parameter -ContentTypeGroups
`
$sourceTemplate = Get-PnPSiteTemplate -OutputInstance -Connection $pnpSourceConnection -Handlers Fields,ContentTypes,Lists
Invoke-PnPSiteTemplate -InputInstance $sourceTemplate -Connection $pnpTargetConnection -OverwriteHandlers ["Fields","ContentTypes","Lists"]

`

How I do it today using Set-PnPField

I have created my own scripts using PnP PowerShell cmdlets, using Get-PnPField, Get-PnPContentType and Get-PnP-List.
Then I filter them using group names so only the ones my team has created is stored in a variable.
Then I test the target Site for existing fields, content-types and lists, and I group them by "Create, Update and Delete".
Then my script can create the new ones, update the existing ones and even delete obsolete ones.

Start by tracing Invoke-PnPSiteTemplate and its Fields, ContentTypes, and Lists handlers in the PnP provisioning engine. Review how existing fields, content types, and lists are handled and how Get-PnPSiteTemplate selects handlers. Done means an existing target site can receive the requested updates without duplicate-field failures, with overwrite and field-group filtering behavior defined and covered by tests.