Try feathering the edge of your selection before copying it  (Select > Modify > Feather in my version of Photoshop)

but here's a few random thoughts.

 In Affinity Photo

Always use transparent background.

Use PNG files

As stated by CNscale go in tight and soften or trim the edges of image.

My suspicion is that you are seeing the borders of images when you reprint........

Quick edit

Your problem may also be related to exposure changes at the edges of your image being cloned.

 This is a quick and dirty example

Top image.........basic building block

Middle image.......a pair of basic blocks set edge to edge. Notice the line due to darker image on the sides of the basic building block.

Bottom image........This is the middle image with a slice of the lighter toned brick  placed on top of the dark line then using a texture erase brush to blend the splice edges into the main image.

Wow, nicely done.  I need to give this a try as I imagine it needs some practice.  thanks.

Hello,

Are you familiar with the images available at Textures.com? 

Brick Category

With a free registration you can download a limited number of "textures" per day. Some are labeled "seamless" and can be blended easily using a vector-based graphics program.

I use these frequently to produce structure interiors and carpeting, roofing and background scenes. I recently bought Affinity Designer but haven't used it yet. I usually use CorelDRAW and print from there. Sometimes I clean up an image in Photoshop before importing it into Corel.

Good Luck, Ed

In Affinity, use the blemish removal tool (looks like a band aid). Enlarge the brush size to cover the area that needs corrected. Then just click along the area and the color will be corrected. The tool can remove other things in images as well.. 

Here is Ricks middle Photo using the Blemish tool. 

Do watch for repeating patterns that stand out, though.

If you look at the bricks for a moment, you can see a few bricks that stand out, and they're obviously repeated.

Take a couple extra minutes with the Affinity Photo Inpainting brush and paint over those. The result is a big wall without obvious repeating patterns ... very nice.