There is no known way to detect photons in flight.
You can only detect a photon when it is absorbed. And not always then.
In theory you might be able to detect a photon when it is emitted, by the effect on the source that emits it. That is usually not possible, usually you tell that photons were emitted by absorbing some of them.
Here is a question -- are photons real, or are they an artifact of detection?
What if light is waves, and it travels through space exactly like waves but when it interacts with molecules (which are quantized, it can't interact with 1.72 molecules), then the result is quantized because the molecules are quantized?
What if molecules always absorb a quantum amount of radiation, leaving behind whatever they don't absorb? What if they radiate a quantum amount of radiation, and no more? (They do.)
Then light would be a wave, and everything we see as photons would happen because of our limited detection methods.
Is that true? I don't know. I can't think of an experiment to test it.
In practice we use whatever theory is most convenient. For geometric optics we assume that light is rays that travel in straight lines. That's most convenient.
For looking at where light actually goes, when diffraction matters, we use wave theory. Because that's most convenient.
When we want look at how light interacts with molecules, we use QED which gives us particles which travel exactly like waves, because that fits easiest into the rest of modern theory. Light as waves or as modern quanta both work perfectly with the data, and we use the version that is most convenient. Light as straight rays does not work as well sometimes. But sometimes it's the simplest and easiest to use.
If two photons are detected at the same time, maybe they are the same wave getting detected twice? So you turn down your emitter until it emits around one photon per second. You put a detector behind each of two slits. And you consider it a double detection if two photons are detected within one nanosecond of each other, which (let's say) is the limit of your ability to time photon detection. Then you can expect to have two photons detected at the same time by accident, because you created two photons around the same time, about once in 10^9 seconds, assuming that photons are produced at random.
On the other hand, suppose that you have your emitter turned down until the wave intensity produces on average one random detection event per second. It could get a detection any time, but the wave is so weak that they are rare. Then the chance that the wave will result in two detections at once is about once in 10^9 seconds.
But imagine you could detect every time your source emits a photon that heads in the right direction. You never get a false positive, when there was no photon emitted but one was detected. You might find that most of the photons are not detected. Maybe you never get two detections when there was only one photon emitted. Or maybe sometimes you do get two detections when there was only one photon emitted.
That would be a great experiment! If you do get two detections, it shows the photon travels through both slits (or that the wave of course did.) If you never get that, then each photon only travels through one slit and the fact that it could have gone through the other one instead bud didn't, somehow affected it. That would be weird and exciting.