ScriptLayerDuper.cs

using System;
using UVtools.Core;
using UVtools.Core.Extensions;
using UVtools.Core.Scripting;

namespace UVtools.ScriptSample;

public class LayerDuper : ScriptGlobals
{
    readonly ScriptNumericalInput<ushort> NumClonesInput = new()
    {
        Label = "Duplicates",
        Unit = "layer(s)",
        Minimum = 1,
        Maximum = 1000,
        Increment = 1,
        Value = 10,
        DecimalPlates = 0
    };

    readonly ScriptNumericalInput<ushort> EveryLayerInput = new()
    {
        Label = "Every",
        Unit = "layer(s)",
        Minimum = 1,
        Maximum = 1000,
        Increment = 1,
        Value = 2,
        DecimalPlates = 0
    };

    public void ScriptInit()
    {
        Script.Name = "Layer Duper";
        Script.Description = "duplicate layers";
        Script.Author = "kamicane";
        Script.Version = new Version(0, 1);

        Script.UserInputs.Add(NumClonesInput);
        Script.UserInputs.Add(EveryLayerInput);
    }

    public string? ScriptValidate()
    {
        return null;
    }

    public bool ScriptExecute()
    {

        int numClones = NumClonesInput.Value;
        int sliceIndex = 0;
        for (int i = 0; i < numClones; i++) {
          var clone = SlicerFile[sliceIndex].Clone();
          SlicerFile.Insert(sliceIndex, clone);
          sliceIndex+=EveryLayerInput.Value + 1;
        }
        // return true if not cancelled by user
        return !Progress.Token.IsCancellationRequested;
    }
}

Calling SlicerFile.Insert in a loop is not efficient due it's allocation and rebuild need.
Consider using SlicerFile.ReallocateStart(numClones) before loop, and then use SlicerFile[sliceIndex] = clone

The reallocate method will not rebuild properties nor recompute layer Z pos, to attain that effect call after the loop: SlicerFile.RebuildLayersProperties()

Edit: However that solution will only work if EveryLayerInput.Value == 1

To be most efficient and work with any EveryLayerInput.Value do:

public bool ScriptExecute()
{

    int numClones = NumClonesInput.Value;
    int sliceIndex = 0;
    var layers = SlicerFile.ToList();
    for (int i = 0; i < numClones; i++) {
      var clone = SlicerFile[sliceIndex].Clone();
      layers.Insert(sliceIndex, clone);
      sliceIndex+=EveryLayerInput.Value + 1;
    }
    
    SlicerFile.Layers = layers.ToArray();
   
    // return true if not cancelled by user
    return !Progress.Token.IsCancellationRequested;
}

Alternative, but previous still better:

public bool ScriptExecute()
{
    int numClones = NumClonesInput.Value;
    int sliceIndex = 0;
    SlicerFile.SuppressRebuildPropertiesWork(() =>
    {
        for (int i = 0; i < numClones; i++) {
          var clone = SlicerFile[sliceIndex].Clone();
          SlicerFile.Insert(sliceIndex, clone);
          sliceIndex+=EveryLayerInput.Value + 1;
        }
    }, true);
    // return true if not cancelled by user
    return !Progress.Token.IsCancellationRequested;
}

That will take a lot of overhead if loop is large