The truth is, if you're underneath a beach or there's too much gravel for you to dig, simply take out your torch and mine the block of sand/gravel and quickly place a torch in the place of that block you just mined. Now, kick back and relax as the torch destroys those pesky blocks of sand/gravel.
There is also a "theory" that the torches in the game itself can hold any block placed above it so it comes handy most of the time. Slap that thumbs up button if you hate creepers and add me on PSN if you want to play Minecraft with me sometime: SquiirtleZ -Modder_Dude0_0
If you want to duplicate Iron/Gold/Diamond blocks then you're in luck. First you might want to mine for 9 of these materials to get either 9 iron/gold ingots or 9 diamonds. Once you get these, go back to your safe place and craft a block out of those 9 items you just got. Now that you have this block of iron/gold/diamond, place it down. Make sure your pickax is the only thing you're carrying and everything else is put away in your inventory. Now, mine the block, press R1 and place down the block before it is even equipped. You should see that you are holding a copy of the block you just mined but the original is still on the ground. Press Square to bring up the crafting options and press L1 to go to decorations and turn the iron/gold/diamond block into ingots/diamonds. It might take a few tries but you will get it eventually. Slap that thumbs up button if you like dupes and add SquiirtleZ on PSN if you want to play GTA V/Minecraft with me. -Modder_Dude0_0