The three conditions
The existing tiles must be firmly bonded across the whole area, the substrate behind them must be sound and dry, and the added thickness must not cause a problem at doors, windows, sockets or the shower tray junction. Tap-test every tile: a hollow ring over more than a small isolated area means stripping back.
Weight and thickness
Every layer adds load. Two layers of tile plus two beds of adhesive on plasterboard can exceed what the board is rated to carry, particularly with large-format porcelain. Thickness also swallows window reveals and pushes tiles proud of sockets and trims, which looks like an afterthought because it is one.
Preparation that makes it work
Where tiling over is appropriate, the old surface must be degreased, keyed or primed with a suitable bonding primer, and any loose tiles cut out and made good flush. Existing silicone must be removed completely, not tiled against.
When to strip back regardless
Always strip back in a shower area where the waterproofing is unknown or older than around a decade, wherever there is any sign of damp behind, and where the existing tiles sit on failed or artex-covered plaster. The saving from tiling over is a day of labour and a skip - a fraction of the cost of the wall coming off later.