There are a couple ways to copy and paste a stroke in Concepts.
To copy an item within your Concepts drawing:
1) Select your stroke or multiple strokes.
2) Tap the duplicate button on the Selection Popup to create an exact copy. It will appear selected and ready for you to edit or move about the canvas.
3) Tap to set it in place.

To copy / paste an item via your device clipboard:
1) Select your stroke or multiple strokes. Tap the paperclip icon to paste it to your device’s clipboard. You can also copy text or images from outside software and import them into the app by using Apple's "copy" function.
2) Tap+hold anywhere on the canvas outside your selection box to bring up a clipboard icon.
3) Tap this icon to paste your selection to the canvas. Alternatively, you can import what is on your device clipboard through the Import menu.
As your device clipboard is accessible in multiple apps, you can copy and paste your selection to outside apps like email or text, or use it to transfer selections between other drawings in Concepts.