There are a couple ways to copy and paste a stroke in Concepts.
1) Select your stroke or multiple strokes (read about Selection here), then touch the duplicate button on the selection popup to create an exact copy. It will appear selected and ready for you to move it about the canvas. Tap to set it in place.
2) Alternatively, you can select an object and paste it to your device’s clipboard by tapping on the paperclip button on the selection popup, or by dragging it to the paperclip under the Import menu (at the top of the object library). When you have a selection pasted to the clipboard, tap+hold anywhere on the canvas to bring up a clipboard icon. Tap this icon - or find the paperclip in any of the clipboard locations in app - and tap it to paste your selection to the canvas. As your clipboard is accessible in multiple apps, you can copy and paste your selection to outside apps, or use it to transfer selections between other drawings in Concepts.