Example of how a spawner could skew the chances for objects to spawin in favour of the player, making it more likely to give the objects a player doesn't have while still respecting a base chance.

program.cs

public class Program
{
    public static void Main()
    {
        // Setup data
        var butterfly = new Spawnable("Butterfly");
        var bumblebee = new Spawnable("Bumblebee");
        var cockroach = new Spawnable("Cockroach");

        var spawnPoint = new SpawnPoint();
        spawnPoint.AddItem(butterfly, 0.70);
        spawnPoint.AddItem(bumblebee, 0.285);
        spawnPoint.AddItem(cockroach, 0.015);

        var inventory = new Inventory();
        //inventory.AddItem(butterfly, 15);
        //inventory.AddItem(bumblebee, 6);
        //inventory.AddItem(cockroach, 1);

        // Use the spawner to spawn an item
        // Despite the cockroach having a 1.5% chance of spawning, the amount of butterflies and bumblebees
        // raises its chance to roughly 6%.
        var spawner = new Spawner();

        for (int i = 0; i < 10000; i++)
        {
            var spawnedItem = spawner.SpawnItem(spawnPoint, inventory);
            inventory.AddItem(spawnedItem);
        }

        Console.WriteLine($"Spawned {inventory.GetItemCount(butterfly)/100.0}% butterflies");
        Console.WriteLine($"Spawned {inventory.GetItemCount(bumblebee)/100.0}% bumblebees");
        Console.WriteLine($"Spawned {inventory.GetItemCount(cockroach)/100.0}% cockroaches");
    }
}

/// <summary>
/// Represents a spawnable item.
/// </summary>
public class Spawnable
{
    public string Name { get; }

    public Spawnable(string name)
    {
        Name = name;
    }
}

/// <summary>
/// Represents a spawn point that holds items with their base chances.
/// </summary>
public class SpawnPoint
{
    private readonly List<(Spawnable Item, double BaseChance)> _items = new List<(Spawnable, double)>();

    public IReadOnlyList<(Spawnable Item, double BaseChance)> Items => _items;

    public void AddItem(Spawnable item, double baseChance)
    {
        _items.Add((item, baseChance));
    }
}

/// <summary>
/// Holds the player's inventory counts.
/// </summary>
public class Inventory
{
    private readonly Dictionary<Spawnable, int> _itemCounts = new Dictionary<Spawnable, int>();

    public void AddItem(Spawnable item, int count = 1)
    {
        if (_itemCounts.ContainsKey(item))
            _itemCounts[item] += count;
        else
            _itemCounts[item] = count;
    }

    public int GetItemCount(Spawnable item)
    {
        return _itemCounts.TryGetValue(item, out var value) ? value : 0;
    }
}

/// <summary>
/// Handles the logic of spawning an item from a spawn point, adjusted by inventory.
/// </summary>
public class Spawner
{
    private const double SkewFactor = 2.0;
    private readonly Random _random = new Random();

    public Spawnable SpawnItem(SpawnPoint spawnPoint, Inventory inventory)
    {
        var skewedChances = ComputeSkewedChances(spawnPoint, inventory);
        return SelectItem(skewedChances);
    }

    private List<(Spawnable Item, double Chance)> ComputeSkewedChances(SpawnPoint spawnPoint, Inventory inventory)
    {
        return spawnPoint.Items
            .Select(pair =>
            {
                var (item, baseChance) = pair;
                var ownedCount = inventory.GetItemCount(item);
                var factor = SkewFactor / (ownedCount + SkewFactor);
                return (item, baseChance * factor);
            })
            .ToList();
    }

    private Spawnable SelectItem(List<(Spawnable Item, double Chance)> probabilities)
    {
        var chanceSum = probabilities.Sum(item => item.Chance);
        var rand = _random.NextDouble() * chanceSum;
        var index = 0;

        while (rand > probabilities[index].Chance)
        {
            rand -= probabilities[index].Chance;
            index++;
        }

        return probabilities[index].Item;
    }
}

If collecting and not using the spawnabales (as resources), the drop rate essentually flattens out. Making it spawn 10k items starting with none and picking them all, you get these results:

Spawned 56,12% butterflies
Spawned 35,92% bumblebees
Spawned 7,96% cockroaches

As you can see, the rarest item (cockroaches) went up from 1.5% to nearly 8%. In games, players usually use up resources they get, so in practise the rarer drops will still be lower than this, while still skewed in their favour.